SUPERLATTICE TUNNELING DETECTORS ...

SUPERLATTICE TUNNELING DETECTORS OPERATING AT λ = 10 µm, BASED ON QUANTUM WELL INTERSUBBAND ABSORPTION B. Levine, K. Choi, C. Bethea, J. Walker, R. Malik

To cite this version: B. Levine, K. Choi, C. Bethea, J. Walker, R. Malik. SUPERLATTICE TUNNELING DETECTORS OPERATING AT λ = 10 µm, BASED ON QUANTUM WELL INTERSUBBAND ABSORPTION. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-611-C5-614. .

HAL Id: jpa-00226716 https://hal.archives-ouvertes.fr/jpa-00226716 Submitted on 1 Jan 1987

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JOURNAL DE PHYSIQUE Colloque C5, suppl6ment au noll, Tome 48, novembre 1987

SUPERLATTICE TUNNELING DETECTORS OPERATING AT QUANTUM WELL INTERSUBBAND ABSORPTION B.F. LEVINE, K.K.

CHOI, C.G.

X

= 10 pm,

BASED ON

BETHEA, J. WALKER and R.J. MALIK

A T and X Bell Laboratories, Murray Hill, NJ 07974, U.S.A.

We demonstrate a novel 10.3 p m superlattice infrared detector based o n doped q u a n t u m wells of GaAs/AlGaAs. Intersubband resonance radiation excites an electron from t h e ground s t a t e into the first excited s t a t e , where it rapidly tunnels o u t producing a photocurrent. W e achieve a narrow bandwidth (10%) photosensitivity with a responsivity a s large as 1.9 A / W and an estimated speed of 30 ps. ~ e c e n t l ~we ' have demonstrated t h e first 10pm infrared detector based on a superlattice of doped GaAs/AlGaAs q u a n t u m wells. T h e motivation for this work is t h a t t h e fabrication of 10pm infrared detectors from 111-V materials would allow advantageous use of their more highly developed growth and processing technologies, as compared with 11-VI ~ o m ~ o u n d s ~F-u~r t.h e r more, device parameters (e.g. band gap, operating temperature, bandwidth , and speed) can be tailored in ways t h a t are difficult t o do with either 11-VI's o r extrinsic S i detectors. W e report here t h e demonstration of a novel high-speed infrared detector based on intersubband absorption and sequential resonant tunneling in doped Gaks/AI,Gal-,As quantum welt superlattices. W e achieved a responsivity as large a s 1.9 AJW a t A = 10.3 p m , a narrow bandwidth response of A X/X = lo%, and estimate t h e speed t o be z 30 ps. From our experiments we have d e t e r ~ i n e d t h a t the mean free p a t h of t h e photogenerated h o t electrons through t h e superlattice is 4500 A. In order t o understand the operation of this detector it is useful t o first discuss tunneling5-' and intersubband absorptiong-'' in doped superlattices T h e infiared radiation is in a superlattice of doped absorbed via t h e q u a n t u m well intersubband resonanceg-'' Ga.4s/AIxGal-,As quantum wells and t h e photoexcited electrons rapidly tunnel o u t of t h e well, thereby producing a photocurrent. W e have studied in detail8 t h e transport characteristics of these weakly coupled GaAs-GaAIAs multiquantum wells and concluded t h a t t h e stair-like potential profile as shown in Fig. ( l a )

Fig. 1 (a) Sequential resonant tunneling in high field domain (right-hand side): tunneling through ground s t a t e (left-hand side). (b) Photoconductivity produced by absorption of intersubband radiation followed by tunneling o u t of well.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19875131

JOURNAL DE PHYSIQUE

C5-612

is the stable configuration when t h e applied bia voltage CV,)is less t h a n m(E2-El), where m is the number of periods of in t h e structure, and El and E2 are t h e energies of t h e ground and t h e first excited s t a t e s in t h e wells respectively. A t low temperatures without infrared radiation, electric conduction is via sequential resonant tunneling either between t h e ground s t a t e s of each well or between t h e ground s t a t e s and t h e first excited s t a t e s of,the adjacent wells, depending on t h e is incident on t h e sample, voltage drop across t h e period. When infrared energy equal electrons are excited t o t h e first excited s t a t e , producing hot electrons after tunneling o u t of t h e wells (see Fig. (lb). Sin& t h e mobility of t h e hot electrons is different from those in t h e wells, a change of conductance is expected. W e named this device a STAIR detector (an acronym for Superlattice Tunneling and Absorption by Intersubband Resonance detector and also because of the stair-like band configuration). In order t o t e s t t h e dependence of the photeexcited tunneling on t h e height and thicktunneling barriers two samples were grown and measured. Sample A conness of t h e A1,Gal-,As sist$d of a 50 period sliperlattice of 65 A GaAs wells (doped 1 . 4 ~ 1 0 ' ~ c r n - ~an$ 95A Ale 2sGao75AS barriers, sandwiched between highly doped contact layers; sample B had 7 0 A quantum wells and 140 A A l o , 3 6 G ~ . s 4 A Sbarriers. These thicknesses and compositions were chosen t o produce only two s t a t e s in the well with an energy spacing close t o 1 0 p m . In crder t o measure the resonance energy and oscillator strength we ~ e r f o r m e dFourier transform interferometer absorption measurementsg-lo with t h e crystal a t Brewster's angle Ob=73', since t h e polarization selection rule of this transition requires t h e optical electric field t o have a component perpen dicular t o the superlattice. T h e absorption of sample A (Fig. 2) is peaked a t 920'cm-' with a full width a t half-maximum of ~ v = 9 7 c m - l corresponding t o an excited s t a t e lifetime of T2= (ii~v)-1=1.1x10-13s(110fs). T h e peak absorbance A = -log (transmission) = 2 . 2 ~ 1 0 - ~ corresponds t o an oscillator s t r e r ~ g t h ~of- ~f=0.6 ~ in good agreement with o u r theoretical value f=0.8. In order t o measure t h e infrared photoconductivity a detector was fabricated by etching a 5 0 p m - d i a m mesa and making Ohmic contact t o t h e top and bottom nt -GaAs layers. In addition, a 45' angle was polished on t h e substrate (see insert in Fig. 2) to allow t h e infrared light t o back illuminate t h e detector a t a 45' angle of incidence.

to$^^-^^

T h i s allows for a large optical field normal t o the superlattice. T h e strongly resonant character of the photocurrent (Fig. 2) is in close agreement with the measured absorption spectrum: Furthermore, a s expected, t h e photosignal was determined t o be highly polarized with t h e optical transition dipole moment aligned normal t o t h e superlattice. T h e responsivity R of Sample A increased with bias, reaching a maximum value of R=0.5 A/W a t a bias of 2.6V, corresponding t o a quantum efficiency of 6%. By fitting this d a t a (shown in Fig. 3)

R(OTUSR(SITIVE LENGTH

f (PERIQOSI

PHOTON ENERGY [cm"l

Fig. 3 Solid points are measured responsivity Fig. 2 Curve is measured absorption spectrum for Sample A; solid points are photocurrent vs. photon energy (normalized t o the peak absorbance); insert shows device geometry

R vs. the photosensitive length 1 of t h e high field domain for Sample A; t h e curve is theory including t h e hot electron mean free path L.

with a theoretical calculation1 which includes t h e responsivity dependence o n t h e photoexcited tunneling probability a n a t h e m$an free path L we can determine both of these quantities. T h e result is p=60% and L=2500 A. This value for L is much longer t h a n a ballistic mean free path12~13due t o the rapid potential drop produced by the resonant alignment of El and E2. If we a t t e m p t t o increase t h e responsivity further in Sample A by increasing t h e bias V, the d a r k current rapidly increases. However, due t o the thicker tunneling banners in Sample B, t h e dark current is several orders of magnitude lower for t h e same bias and hence t h e voltage can then be increased further, thereby increasing the photoexcited tunneling probability and photocurrent. A t Va=9V, we achieve a high responsivity of R=l.SA/W, (which is $early four times larger than for Sample A) and a long hot electron mean free p a t h of L= 4500A, a t Va=3.4V (nearly twice as large as Sample A). F o r biases larger than 3.4V, t h e mean free p a t h , L, rapidly increases s o t h a t t h e effective photoconductive transport distance, becomes the total superlattice thickness. W e have developed a detailed theory of t h e intersubband absorption, photoexcited tunneling and hot electron transport through these multiquantum well superlattices. A s shown in Fig. 4 o u r calculations are in good agreement with experiment over 3 orders of magnitude in responsivity. F r o m this d a t a we can also estimate the electron velocity and hence estimate the response speed t o be very fast (30 psec). In conclusion we have demonstrated a new concept in long wavelength infrared detec= tors, a n d achieved a high responsivity R=l.SA/W, an advantageous narrow bandwidth (A lo%), : and a very high estimated response speed (30 psec).

Fig. 4 T h e voltage dependence of t h e dark current and t h e responsivity of Sample B. T h e dashed curve is the theoretical fitting t o the responsivity.

REFERENCES [I] B. F. Levine, K . K. Choi, C. G. Bethea, J. Walker and R. J. Malik, Appl. Phys. L e t t 50, 1092(1987). [2] J. S . Smith, L. C . Chiu, S. Margalit, A. Yariv, and A. Y. Cho, J. Vac, Sci. Technol. B 1, 37 6(1983)

(31 D. D. Coon and R. P. G. Karunasiri, Appl. Phys. Lett. 45, 649(1984). 141 F. Capasso, K. Mohammed, a n d A. Y. Cho, IEEE J. Quantum Electron. QE-22, 1853(1986). 151 L. Esaki and L. L. Chang, Phys. Rev. Lett. 33, 495(1974)

C5-614

JOURNAL DE PHYSIQUE

Y. ICawamura, K. W a k i t a , H. Asahi, and I