Variable Temperature Carrier Dynamics in Bulk (In

Variable Temperature Carrier Dynamics in Bulk (In)GaAsNSb Materials Grown by MOVPE for Multi-junction Solar Cells Yongkun Sina,*, Zachary Lingleya, Stephen LaLumondierea, Nathan Wellsa, William Lotshawa, Steven C. Mossa, Tae Wan Kimb, Luke J. Mawstb, and Thomas F. Kuechc a Electronics and Photonics Lab, The Aerospace Corporation, El Segundo, CA 90245 b Electrical and Computer Engineering Dept., c Chemical and Biological Engineering Dept. University of Wisconsin – Madison, Madison, WI 53706 ABSTRACT III-V multi-junction solar cells are typically based on a triple-junction design that consists of an InGaP top junction, a GaAs middle junction, and a bottom junction that employs a 1 – 1.25 eV material grown on GaAs substrates. The most promising 1 – 1.25 eV material that is currently under extensive investigation is bulk dilute nitride such as (In)GaAsNSb lattice matched to GaAs substrates. The approach utilizing dilute nitrides has a great potential to achieve high performance triple-junction solar cells as recently demonstrated by Wiemer, et al., who achieved a record efficiency of 43.5% from multi-junction solar cells including MBE-grown dilute nitride materials [1]. Although MOVPE is a preferred technique over MBE for III-V multi-junction solar cell manufacturing, MOVPEgrown dilute nitride research is at its infancy compared to MBE-grown dilute nitride. In particular, carrier dynamics studies are indispensible in the optimization of MOVPE materials growth parameters to obtain improved solar cell performance. For the present study, we employed time-resolved photoluminescence (TR-PL) techniques to study carrier dynamics in MOVPE-grown bulk dilute nitride InGaAsN materials (Eg = 1 – 1.25 eV at RT) lattice matched to GaAs substrates. In contrast to our earlier samples that showed high background C doping densities, our current samples grown using different metalorganic precursors at higher growth temperatures showed a significantly reduced background doping density of ~ 1017 /cm3. We studied carrier dynamics in (In)GaAsNSb double heterostructures (DH) with different N compositions at room temperature. Post-growth annealing yielded significant improvements in carrier lifetimes of (In)GaAsNSb double heterostructure (DH) samples. Carrier dynamics at various temperatures between 10 K and RT were also studied from (In)GaAsNSb DH samples including those samples grown on different orientation substrates. Keywords: Dilute nitride, MOVPE, carrier dynamics, TR-PL, multi-junction solar cell, defects, traps 1. INTRODUCTION Bulk dilute nitride materials with a 1 – 1.3 eV band-gap that are lattice matched to GaAs substrates are attractive for high efficiency multi-junction solar cells. Understanding carrier dynamics in these materials is crucial in optimizing material growth, but only a small number of groups have reported carrier lifetimes of MBE-grown bulk InGaAsNSb materials [2] and of bulk InGaAsNSb materials grown by MOVPE [3, 4]. It is well known that MOVPE is a preferred technique in producing multi-junction solar cells based on III-V materials. Issues related to performance including low internal quantum efficiencies (IQEs) and photocurrent mismatch between individual cells have been correlated to the amount of N (~2 – 3%) required to achieve a 1eV band-gap in InGaAsN material [2]. Post-growth thermal annealing improves the IQEs [3 − 5], but high background carbon concentrations observed in MOVPEgrown materials have been correlated with short minority carrier diffusion lengths. One group has recently shown in their MBE-grown materials that the addition of dilute amounts of Sb to strained InGaAsN can increase the IQE of lattice-matched bulk materials used in solar cells [2]. However, the incorporation of N is difficult in MOVPE-grown materials with dilute amounts of Sb [5, 6] due to either possible change in the surface chemistry with Sb acting as a surfactant or gas phase reaction involving the Sb source. As a surfactant, the accumulation of Sb at the surface competes with the N for available bonding sites, limiting N incorporation into the solid phase [5]. At this time, the effects of Sb on MOVPE-grown InGaAsN materials are not well understood. In the present study, we report carrier lifetime measurements of MOVPE-grown bulk dilute nitride materials with an ~ 1 − 1.3 eV band-gap. _______________________________________________________________________ *[email protected]; phone (310) 336-1734; fax (310) 563-5755

Physics, Simulation, and Photonic Engineering of Photovoltaic Devices III, edited by Alexandre Freundlich, Jean-François Guillemoles, Proc. of SPIE Vol. 8981, 89811A · © 2014 SPIE CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2037385 Proc. of SPIE Vol. 8981 89811A-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/09/2014 Terms of Use: http://spiedl.org/terms

2. EXPERIMENTAL METHODS MOVPE Dilute nitride samples were grown by the University of Wisconsin at Madison (UW-M) using a vertical chamber MOVPE reactor with a close-coupled showerhead gas delivery system as previously described [7]. Trimethylindium (TMIn) and trimethylgallium (TMGa) were used as group III precursors, tertiary butyl arsine (TBAs) and dimethylhydrazine (DMHy) were used as As and N sources, and trimethylantimony (TMSb) was used as an Sb precursor. The growth temperature of 600 ºC, which is higher than previously used, and the growth rate of 1.92 μm/hr were used to grow bulk dilute nitride materials. High-resolution X-ray diffraction (HR-XRD) and ω-2θ rocking curves around the (004) reflection were used to determine the out-of-plane lattice parameter of the dilute nitride films. Secondary ion mass spectroscopy (SIMS) was used to determine group III and group V mole fractions of the selected dilute nitride materials as well as the background oxygen and carbon concentrations. Thermal annealing was performed using rapid thermal annealing (RTA) to improve photoluminescence (PL) intensity. According to our previous study, the optimal condition of the RTA was at 800ºC for 30 sec [6]. Van der Pauw Hall measurements were used to determine the mobility and background hole concentrations at 300 K for samples grown on semi-insulating GaAs (100) substrates. Continuous wave (CW) photoluminescence (PL) spectra at low temperatures (30 − 250K) were measured using the 488 nm emission of an Ar laser with an M1500 Jobyn-Yvon Spex spectrometer and a liquid-nitrogen cooled Ge detector. TR-PL Time-resolved photoluminescence (TR-PL) techniques were employed to measure carrier lifetimes from dilute nitride samples. Carrier dynamics were studied by TR-PL at various temperatures. Selected areas were excited by laser pulses from a mode-locked Ti:Sapphire laser with a pulse duration of 120 fsec and a repetition rate of 200 kHz. The excitation wavelength was varied and a Hamamatsu streak camera with a resolution of ~30 ps was used for the PL detection at room temperature. Low temperature (LT) TR-PL utilized laser excitation through a modelocked Ti:Sapphire oscillator operating at 780 nm with a pulse duration of ca. 100 fs and a repetition rate that was varied by an acousto-optic (AO) pulse picker. PL spectra were measured with an Acton 0.250 m monochromator. LT TR-PL was measured with the combination of an InGaAs (or Si) APD and a Becker & Hickl SPC-130 timecorrelated single photon counting (TCSPC) board, whereas LT steady-state PL was detected with a Hamamatsu R5509-79 PMT. DLTS DLTS samples were prepared by fabricating p-n junction diodes. Photolithography was employed to define patterns for p-Ohmic contact that consisted of Ti-Pt-Au using lift-off process. AuGe-Ni-Au was evaporated for backside n-Ohmic contact. The samples were alloyed in RTA at 450 ºC for 30 sec. A digital deep level transient spectroscopy (DLTS) was used to study trap characteristics in dilute nitride materials by measuring transient capacitance. FIB and TEM A focused ion beam (FIB, FEI Strata 400) was employed to prepare TEM cross sections from dilute nitrides grown on different orientation substrates. A JEOL high-resolution transmission electron microscope (HRTEM) was employed for TEM imaging. 3. EXPRIMENTAL RESULTS AND DISCUSSION Dilute Nitrides Samples used in the present study were grown by UW-M using MOVPE. A number of bulk dilute nitride samples were grown to study the effects of growth temperature and different substrate orientation on energy band-gaps and PL intensities. Growth conditions used to grow lattice-matched dilute nitride materials with a RT energy band-gap of 1.29 eV included a growth temperature of 600 °C, a growth pressure of 150 torr, a V/III ratio of 139.2, and a growth rate of 1.5 µm/hr. The lattice matching condition was DMHy/TBAs = 15.32, Sb/(Sb+As) = 0.0028, and TBAs/III = 8.50. Hall measurements of (In)GaAsNSb grown at 150 torr showed a p-type background density of 1.5×1017 cm-3 and the mobility of 169 cm2/V⋅s.

Proc. of SPIE Vol. 8981 89811A-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/09/2014 Terms of Use: http://spiedl.org/terms

Several research groups have reported that post-growth thermal annealing significantly improves optical qualities of (In)GaAsN(Sb) materials [3 − 5]. Thermal annealing was performed in the MOVPE reactor for some samples. A two stage annealing process was employed as reported in our previous publications [3, 4]. Some of our dilute nitride samples were also annealed using RTA. Comparable improvements in PL intensities were observed from both MOVPE and RTA annealed samples. Nearly lattice-matched material (InGaAsN, Sample 1) with a RT energy band-gap of 1.25 eV was grown at 600 °C and 150 torr, III/V ratio of 64.73, and growth rate of 1.92 µm/hr. The lattice matching condition was In/(In+Ga) = 0.052, DMHy/TBAs = 9.26, and TBAs/III = 6.31. We were unable to perform Hall measurements on 0.3 µm-thick InGaAsN films grown under these conditions because the dilute nitride layers were fully depleted and the layers were highly resistive. Figure 1 shows room temperature PL spectra (a) and HR-XRD (b) measured from the InGaAsN sample.

15

HRXRD

0

900

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1100

1200

01

1000

960

+900

1000

a..o.n1e.1. I+

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(b)

Figure 1. Room temperature PL spectra (a) and HR-XRD (b) measured from InGaAsN or Sample 1. In addition to Sample 1, (In)GaAsN materials (Samples 2 – 5) with different In molar flows were also grown. Closely lattice-matched materials (Samples 3 – 5) with RT energy band-gaps of 1.12 – 1.17 eV were grown at 600 °C and 150 torr, III/V ratio of 97.92 – 103, and growth rate of 1.92µm/hr. Sample 2 was a control sample grown without In (GaAsN). The lattice-matched conditions of bulk InGaAsN materials were In/(In+Ga) = 0.052 – 0.098, DMHy/TBAs = 15.32, and TBAs/III = 6 – 6.31. Table 1 summarizes growth conditions used for Samples 2 – 5 along with lattice mismatch results. Table 1. Summary of growth conditions and lattice mismatch results. Sample

In /(In+Ga)

DMHy/ TBAs

III/V

Eg (eV)

Lattice mismatch (arc sec)

2 3 4 5

N/A 0.052 0.076 0.098

15.32 15.32 15.32 15.32

108.61 102.99 100.39 97.92

1.072 1.12 1.157 1.168

1518 486 -99 -500

Growth Temperature (°C) 600 600 600 600

Growth Pressure (Torr) 150 150 150 150

Figure 2 shows the HR-XRD (a) and RT PL spectra (b) measured from Samples 2 – 5. Van der Pauw Hall measurements yielded a p-type carrier concentration of 1.4×1017 cm-3 and a mobility of 113 cm2/V⋅s for Sample 4


and a p-type carrier concentration of 5.2×1017 cm-3 and a mobility of 147 cm2/V⋅s for Sample 3. The p-type carrier concentration is attributed to the incorporation of background carbon impurities.

Increase In flow

countsis

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Figure 2. HR-XRD (a) and RT PL spectra (b) measured from Samples 2 – 5. Carrier Lifetimes We studied carrier dynamics in a series of MOVPE-grown bulk dilute nitride materials lattice matched to GaAs substrates including InGaAsNSb layers (Eg = ~ 1.0 – 1.2 eV at RT) as well as GaAsNSb layers. The dilute nitride layers were clad by GaAs layers, forming a DH. Carrier lifetimes were measured from both asgrown and thermally annealed samples at low temperatures and RT. Figure 3 shows steady-state PL spectra measured from InGaAsN DH samples (listed as Sample 5 in Table 1) at various temperatures between 10 and 295 K. The spectra measured from the InGaAsN DH samples on (100) and (311) B GaAs substrates are shown in Figure 3 (a) and (b), respectively. SIMS analyses showed that the layer grown on (100) substrate was In0.0669Ga0.933As0.985N0.0148 and the layer grown on (311)B substrate was In0.0699Ga0.93As0.981N0.0196. PL decay curves were measured from the two samples for the detection wavelengths corresponding to band edge emission. Figure 4 shows PL decay curves measured at three temperatures (10, 30, and 80 K) for detection wavelengths specified in the insert. The PL decay curves were fitted with double exponentials. We were able to estimate carrier lifetimes and amplitudes only from the PL decay curve of the (311)B sample at 10 K. Other measurement results were all IRF limited (~ 350 ps for InGaAs APD). The sample on GaAs (311) B substrate showed a faster component (τ0) of 1.4 ns and a slower component (τ1) of 15.3 ns for the detection wavelength of 1055 nm at 10 K, whereas the sample on GaAs (100) substrate showed a faster component of shorter than 350 ps (IRF limited). Both samples showed lifetimes of shorter than 100 ps at RT.


(a)

(b) Figure 3. Steady-state PL P spectra meaasured from thee InGaAsN DH H sample grow wn on GaAs (1000) substrate (a) and the spectraa measured fro om the InGaAssN DH grown on GaAs (311)B substrate (b). ( PL spectrra were measuured at various tem mperatures betw ween 10 and 295 K.


(311)b and (100) Peak Lifetime Summary 1

0.1

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(311)b 10K 1055nm (311)b 30K 1050nm (311)b 80K 1040nm

A1=0.069 A2=0.734 t1=15.27 t2=1.377

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Figure 4. PL decay curves measured from InGaAsN samples grown on (100) and (311)B GaAs substrates at three temperatures (10, 30, and 80 K) for detection wavelengths given in the insert. To gain insight into different carrier dynamics behaviors observed from the two samples at 10 K, TR-PL was also performed for the detection wavelength of ~ 1450 nm corresponding to deep trap levels. Figure 5 shows full steadystate PL spectra (950 - 1600 nm) measured from the two samples on GaAs (100) and (311) B GaAs substrate at three temperatures (10, 30, and 80 K). The spectra show two dominant peaks: one peak at ~ 1370 nm due to H2O absorption and the other peak at ~ 1450 nm due to deep trap levels. PL decay curves were measured from the two samples at 10, 30, and 80 K. Figure 6 shows transient PL curves measured from the two samples at two temperatures (10 and 80 K) for the detection wavelength of 1450 nm. The PL decay curves were then fitted with double exponentials to estimate carrier lifetimes and amplitudes. Table 2 summarizes our fitting results, where two lifetimes corresponding to faster components (τ0) and slower components (τ1) are given along with amplitudes. At 10 K, the (311)B sample showed a faster component of 36.1 ns, whereas the (100) sample showed a faster component of 48.5 ns. The difference in trap lifetimes is an indication that carrier dynamics in our dilute nitride samples are governed by the Shockley-Read-Hall (SRH) process due to the presence of a number of defects and traps that result in nonradiative recombination of carriers. Although it is crucial to identify dominant traps and defects in dilute nitrides to better understand carrier dynamics in dilute nitrides, defects and traps in these materials systems are not well understood.


Deep Traps T H2O Absoorption

en ej Aruetui 7d

Band Edge E

Figure 5. Steady-state S PL L spectra meassured from diluute nitride sam mples grown on (100) and (311)B GaAs substrates at three tem mperatures (10, 30, and 80 K)).

(311)b and d (100)b Trap p State Lifetim me 1

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c measureed from InGaA AsN samples grrown on (100)) and (311)B GaAs G substratess at 10 Figure 6. Transient PL curves f the detection wavelength of o 1450 nm (trap levels). and 80 K for


Table 2. Summary of carrier lifetimes and amplitudes estimated from PL decay curves of the two structures at 10 and 80 K for detection wavelengths corresponding to deep trap levels.

(311)B at 10 K

(311)B at 80 K

A0 (a. u.)

1143.5

A1 (a. u.)

482.5

τ0 (ns)

A0 (a. u.)

2574.4

A1 (a. u.)

1526.5

36.1

τ0 (ns)

48.5

τ1 (ns)

265.3

τ1 (ns)

244.3

A0 (a. u.)

4311.4

A0 (a. u.)

6358.4

A1 (a. u.)

769.9

A1 (a. u.)

1743.4

τ0 (ns)

17.0

τ0 (ns)

32.7

τ1 (ns)

146.8

τ1 (ns)

209.3

(100) at 10 K

(100) at 80 K

Dislocations To identify the root causes responsible for short carrier lifetimes in bulk dilute nitride layers, a series of TEM cross sections were prepared using FIB to study the presence of dislocations in the structures. Figures 7 and 8 show TEM images obtained from three dilute nitride samples grown on different substrates and orientations. Dilute nitride layers were grown on InGaP that was pre-deposited over different substrates. Figure 7 (a) and (b) are high angle annular dark field (HAADF) and annular dark field (ADF) images from the sample grown on GaAs (100) substrate, respectively, whereas Figure 7 (c) and (d) show ADF images from the sample grown on GaAs (311)B substrate under two different magnifications. As shown in Figure 7, no dislocations were observed from bulk dilute nitrides grown on GaAs (001) and GaAs (311) B substrates.

Dilute Nitride InGaP GaAs (100) substrate 200 nm

200 nm

(a)

(b)


Dilute Nitride InGaP InGaP

GaAs (311) B substrate

GaAs (311) B substrate

(c)

(d)

Figure 7. TEM images from the sample grown on GaAs (100) substrate (a, b) and from the sample on GaAs (311) B substrate (c, d). We also studied TEM cross sections prepared from bulk dilute nitrides grown on Ge substrate, which was grown as a reference sample. The three structures were grown during the same MOVPE growth run. Figure 8 (a) and (b) show ADF and dark field images from the sample grown on Ge substrate under two different magnifications. As can be seen from Figure 8, dislocations were observed from dilute nitrides grown on Ge substrate. Our TEM studies clearly indicate that short carrier lifetimes that we measured from dilute nitride samples at RT are due to point defects, not due to extended defects including dislocations. Point defects that act as traps most likely dominate carrier dynamics through the SRH process.

Dilute Nitride InGaP Ge substrate

1 µm (a)


Dilute Nitride

InGaP

Ge substrate

0.2 µm (b) Figure 8. TEM images from the sample on Ge substrate under two different magnifications. Traps We are currently investigating traps in bulk dilute nitrides using deep level transient spectroscopy (DLTS). For our DLTS samples, doping densities of 1.5×1017 and 5×1017 cm-3 were chosen for a 100 nm thick p-GaAsSbN layer and a 250 nm thick n-GaAsSbN layer, respectively. The structure was fabricated into p-n junction diodes with a typical contact size of 100 × 100 μm2. Dilute nitride diodes typically showed the capacitance of ~ 20 pF. Our DLTS results will be published elsewhere. 4. SUMMARY Bulk, nominally lattice-matched (In)GaAsN(Sb) materials with band gap energies of 1.0 – 1.3 eV were grown on GaAs (100) substrates using MOVPE. We studied carrier dynamics in (In)GaAsN(Sb) DH structures at RT and LT using TR-PL techniques. Bulk dilute nitride materials were also grown on GaAs (311) B substrates. The InGaAsN sample on GaAs (311) B substrate showed a faster component (τ0) of 1.4 ns and a slower component (τ1) of 15.3 ns for the detection wavelength of 1055 nm at 10 K, whereas the InGaAsN sample on GaAs (100) substrate showed a faster component of shorter than 350 ps (IRF limited). Both structures showed a lifetime of < 100 ps at RT. TEM cross sections were prepared from both dilute nitride DH structures grown on GaAs (100) and GaAs (311) B substrates. No obvious dislocations were observed from both structures. Our results suggest that short carrier lifetimes measured from MOVPE-grown dilute nitride materials at RT are most likely due to the presence of a high density of point defects and traps that dominates carrier dynamics via the non-radiative SRH process. ACKNOWLEDGMENTS The work at The Aerospace Corporation was supported by its Sustained Experimentation and Research for Program Applications (SERPA). The work at the University of Wisconsin at Madison was supported by NSF MRSEC (DMR-1121288). The authors are grateful to Miles Brodie for his help in TEM sample preparation. REFERENCES 1. M. Wiemer, V. Sabnis, H. Yuen, “43.5% efficient lattice matched solar cells,” Proc. of SPIE 8108, 810804-1, 2011.


2. D. B. Jackrel, S. R. Bank, H. B. Yuen, M. A. Wistey, and J. S. Harris, “Dilute nitride GaInNAs and GaInNAsSb solar cells by molecular beam epitaxy,” J. Appl. Phys. 101, 114916, 2007. 3. Y. Sin, S. LaLumondiere, W. T. Lotshaw, S. C. Moss, T. Garrod, T. W. Kim, J. Kirch, L. J. Mawst, “Carrier dynamics in MOVPE-grown bulk dilute nitride materials for multi-junction solar cells,” Proc. of SPIE 7933, 79330H, 2011. 4. T. W. Kim, T. J. Garrod, K. Kim, J. J. Lee, S. D. LaLumondiere, Y. Sin, W. T. Lotshaw, S. C. Moss, T. F. Kuech, Rao Tatavarti, and L. J. Mawst, “Narrow band gap (1eV) InGaAsSbN solar cells grown by metal organic vapor phase epitaxy,” Appl. Phys. Lett. 100, 121120, 2012. 5. K. Volz, D. Lackner, I. Nemeth, B. Kunert, W. Stolz, C. Baur, F. Dimroth, and A. W. Bett, “Optimization of annealing conditions of (GaIn)(NAs) for solar cell applications,” J. Cryst. Growth 310, pp. 2222-2228, 2008. 6. T. W. Kim, K. Kim, J. J. Lee, T. F. Kuech, and L. J. Mawst, “Impact of thermal annealing on bulk InGaAsSbN materials grown by metalorganic vapor phase epitaxy,” 55th Electronic Materials Conference (EMC), Next Generation Solar Cell Materials and Devices, South Bend, Indiana, June, 2013. 7. T. Garrod, J. Kirch, T. Kim, J. Konen, L. J. Mawst, and T.F. Kuech, “Narrow band gap GaInNAsSb material grown by metal organic vapor phase epitaxy (MOVPE) for solar cell applications,” J. Cryst. Growth 315, pp. 6873, 2011.
