Novel infrared detector

INFRAREDPHYSICS &TECHNOLOGY ELSEVIER

Infrared Physics & Technology 38 (1997) 9-12

Novel infrared detector Yukun Yang *, Wenming Li, Lei Yu, Xin Sun, Lixing Xu, Lantain Hou Analysis Test and Experiment Center, Jilin University, Changchun 130023, China Received 29 May 1996

Abstract

An n-PbTe epilayer was grown on a p-Si substrate by the hot wall epitaxy (HWE) technique, so an n-PbTe/p-Si heterojunction was produced. Then the heterojunction was utilized to make a mid-infrared detector for the first time, to our knowledge. A detectivity of D* = 1.6 X 10 9 c m Hz 1/2 W-1 could be achieved.

1. Introduction Narrow gap semiconductors are among the most suitable materials for infrared detectors due to their high quantum efficiency, low noise level at given operating temperatures and their band gap which can be tailored to achieve desired cut-off wavelengths. We use I V - V I narrow gap semiconductors rather than mercury cadmium telluride as infrared sensitive narrow gap semiconductor material because growth and fabrication techniques are much easier, and the dependence of the band gap on the composition is much less pronounced, while maximum sensitivities are comparable or even somewhat higher than those of mercury cadmium telluride. Generally, I V - V I semiconductor devices are based on epitaxial films grown on cleaved BaF2(111) substrates due to the close match of the lattice constants and the thermal expansion coefficients. However, a much more elegant approach would be growing a thin epilayer of I V - V I semiconductor material on a silicon wafer heteroepitaxially, and

* Corresponding author.

processing the signal in the silicon substrate, thus producing a truly monolithic device. The use of silicon Schottky barriers for photodetection, especially in the infrared, is well established, but quantum efficiencies are quite low only a few percent, i.e.. Scott et al. [1] point out that an ideal detector similar to the Schottky structure could be fabricated by replacing a Schottky contact by a narrow gap semiconductor, thus creating a heterostructure. Heterostructures which utilize a thin film of narrow gap semiconductor grown on a Si substrate have the potential to function as high-efficiency, easily fabricated infrared detectors. Heterojunctions of PbS with Si have been fabricated [2]. The P b S / S i heterojunctions are characterized by high detectivity in the near infrared wavelength region. The optical and detector properties of the P b S / S i heterojunctions have been reported by Steckl et al. [3]. However, little work has been reported on heterojunctions in which lattice mismatch is quite large. Scott et al. [4] have characterized two such systems, namely P b T e / p - S i and S n T e / p - S i . The structures were prepared by congruently evaporating films of PbTe and SnTe from single sources of the corn-

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Y. Yang et al./ lnfrared Physics & Technology 38 (1997) 9-12

pounds onto a (100)p-Si wafer. They have analyzed the composition and electrical properties of deposited films as well as the electrical and photoresponse properties of the heterojunction. High-quality epitaxial IV-VI layers are obtained on (lll)-oriented Si substrates if a thin intermediate BaF2/CaF 2 or CaF 2 buffer layer is employed [5,6]. Epitaxial growth of PbTe on a Si substrate without an intermediate buffer layer to fabricate a PbTe/Si heterostructure has been undertaken by Vaya et al. [7] and the present authors [8], in which the degree of lattice mismatch is as high as 17% and the thermal expansion coefficient mismatch is up to 10 times. Epitaxial growth of PbSe on (111) and (100)-oriented Si substrate without an intermediate buffer layer was studied by Miller et al. [9]. In view of the prediction by Scott et al., we have fabricated a novel mid-infrared detector with an nP b T e / P - S i heterojunction. In this article, we present the description of growing a n-PbTe epilayer on a p-Si substrate by the hot-wall-epitaxy (HWE) technique, and the fabrication of an n - P b T e / p - S i heterojunction mid-infrared detector, which is unprecedented as far as we know.

2. Analysis of detection process The process of photodetection in a Schottky diode infrared detector may be divided into three steps. In the first step is absorption, photons incident on the detectors interact with the metal film to produce an electron-hole pair. The second step is transport. In this step, holes move from the point of excitation to the Schottky barrier. After this step, holes are either reflected back into the metal film or transmitted over the barrier into the semiconductor material where they can be collected. The inelastic free path of holes in metal is short. This limits the thickness of the active layer to several tens of angstroms. Thus most of the available photons are lost. If a thicker film is used, more photons can be absorbed, but the quantum efficiency is reduced due to inelastic scattering. After absorption, the holes move from the point of excitation to the Schottky barrier. The average lifetime of the carder is extremely short in metal due to hole-hole scattering.

The transmission probability of the Schottky barrier is quite small as a result of poor K-vector matching between the metal and the semiconductor. The above factors limit the quantum efficiency of the Schottky diode infrared detector to a few percent. Scott et al. [1] predicted that an ideal detector similar to the Schottky diode could be fabricated by replacing metal film with the narrow-gap semiconductor PbTe, leading to the heterojunction of nPbTe/p-Si. The n-type doping of PbTe is sufficient for any band bending to be negligible. The photoresponse threshold is the energy difference between the valence band maximum of the Si and the conduction band minimum of PbTe. This structure will minimize the inelastic processes. The band gap of PbTe is less than threshold, which can maximize absorption in the overlayer. In the absorption step, the narrow gap semiconductor will produce more detectable carders than a metal or silicide. The power density of states makes the absorption length in a semiconductor larger than that in a metal, therefore a thicker PbTe layer can be used to absorb more photons. In the transport step, there are few holes near the valence band maximum of n-PbTe, so hole-hole scattering can be neglected. In the transmission step the K ± matching is better than for a Schottky barrier. This will reduce reflections near the threshold. The above factors can result in a substantial increase in quantum efficiency of an n - P b T e / p - S i heterostructure over other Si-based infrared detectors.

3. Infrared detector fabrication An n - P b T e / p - S i heterojunction was formed by growing slightly lead-rich PbTe epitaxiaUy on a ptype silicon substrate using a home-made simplified HWE apparatus [9]. The PbTe source was synthesized from a stoichiometric epitaxy melt of elements with a purity of 6 N. A l0 ~ cm p-type silicon wafer of (100) orientation was used as substrate. The wafer was thoroughly degreased, and then treated with acid to get rid of the metallic impurities. Then it was boiled in a solution of HCI:HEO2:H20-- 1:1:4

Y. Yang et aL / Infrared Physics & Technology 38 (1997) 9-12 PbTe(200)

i

40

35

30

1 i

I

25

20

2 e (degrees)

Fig. I. X-raydiffraction spectrum of n-PbTe/p-Si heterojunction.

1t

good rectification characteristic and low reverse leakage. The differential resistance of the device at zero bias at room temperature is R o = 1.25 × 103 I'L Its active area is A = 0.2 cm 2. We used a 500 K blackbody as radiation source and a filter to obtain 4.26/xm mid infrared radiation. The measured voltage peak responsivity R a of this device for A = 4.26 /~m at room temperature was R a = 964 V / W . The quantum efficiency was then calculated by the equation (1)

77 = R a E ~ / q R o

for 10 min to form a clean oxide layer on the silicon surface. In the final stage of cleaning, the silicon wafer was dipped into a hydrofluoric acid solution for 30 s to get rid of the oxide layer and to form the hydrogen termination of the silicon surface, i.e. a passivation layer. This layer works against oxidation or contaminants even in air. Prior to the growth, this substrate was preheated to over 550°C for 30 min at a working pressure of 8 × 10 -7 Tort to remove the passivation layer, thus a clean silicon surface was obtained. The substrate temperature was reduced to the growth temperature of 390°C, while the source and the wall were then heated and kept at 515°C. A growth rate of 1 /~m/h was obtained for this source and substrate temperature combination. The growth period was 1 h. The PbTe epilayer was confirmed to be n-type by a hot-probe measurement. The single crystallinity of the epilayer was confirmed by X-ray diffraction (see Fig. 1). The carrier concentration of the PbTe epilayer was measured to be 1.2 × 10 L8 cm -3 from the Hall measurement by the Van der Pauw method on the epilayer grown on a BaF 2 (111) substrate under the same conditions used for n - P n T e / p - S i heterojunction preparation. Ohmic contacts were made on the n-PbTe epilayer by soldering a gold wire using indium, and on the silicon substrate by depositing a aluminium spot. Illumination was from the backside through the infrared transparent silicon substrate.

where E~ is the photon energy of detected light, and q is the electron charge. Substituting all data in Eq. (1), the value quantum efficiency was calculated to be 21.2%. The detectivity of this device was calculated by the well known relation qrlh D; =

/ RoA

hc V

(2)

4kT

where "r/ is the quantum efficiency, h is the Planck constant, c is the speed of light, k is the Boltzmann constant, and T = 300 K. The detectivity D * = 1.6 × 109 c m H z 1/2 W - 1 of this device for h = 4.26 /zm mid-infrared radiation at room temperature was calculated by Eq. (2). In order to know the temperature limit below which the device does not work we have used the Model LT-3-1 l0 liquid transfer Heli-Tran refrigerator to decrease the temperature of the detector. We found that it did not work when the temperature dropped below - 3 2 ° C . We think the reason for this effect is that the difference of thermal expansion coefficients between PbTe and silicon is too large. The epilayer would break at such a low temperature.

2.0 1.5

: o

1.0

0.5

4. Detector characteristics

-2.0 -1.5 -I.0 -0.5 i • p I

s 2.0 i Voltage(V)

-0.5

The current-voltage characteristic of the device at room temperature is shown in Fig. 2. It exhibits a

Fig. 2. Current-voltagecharacteristic of n-PbTe/p-Si heterojunction mid infrared detector.

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Y. Yang et al. / lnfrared Physics & Technology 38 (1997) 9-12

5. Conclusion We have fabricated an n - P b T e / p - S i heterojunction mid infrared detector, probably for the first time. As predicted by Scott et al., the novel device structure can result in a substantial increase in quantum efficiency over other silicon based infrared detectors. This kind of mid infrared detector could be easily fabricated and used at ambient temperature. No cooling is required. Our fabrication steps are rather crude, even no surface passivation layer was deposited, and no antireflection coating was applied, so considerable improvements could be made easily.

References [1] G. Scott, D.E. Mercer and C.R. Helms, J. Vac. Sci. Technol. B 9(3) (1991) 1781.

[2] H. Sigmund and K. Berchtold, Phys. Status Solidi 20 (1967) 255. [3] Andrew J. Steckl, Hamman Elabd, Ka-Yee Tam, Shey-Ping Shell and M.E. Motamedi, IEEE Trans. Electron Devices ED-27(1) (1980) 126. [4] G. Scott and C.R. Helms, J. Vac. Sci. Technol. B 9(3) (1991) 1785. [5] H. Zogg and M. H~ppi, Appl. Phys. Lett. 47 (1985) 133. [6] H. Zogg, S. Blunier, A. Fach, C. Maissen and J. Masek, Opt. Eng. 33 (1994) 1440. [7] P.R. Vaya, J. Majhi, B.S.V. Gopalam and C. Dattatreyan, Phys. Status. Solidi A 93 (1986) 353. [8] Yang Yukun, Li Wenming, Yu Lei, Yang Yi, Xu Lixing, Xiong Xin, Wang Shanli and Huang Helan, Chin. J. Semicond. 16(8) (1995) 594. [9] Yukun Yang, Wenming Li, Lei Lu, Yi Yang, Lixing Xu, Xin Xiong, Shanli Wang and Helan Huang, J. Cryst. Growth 165 0996) 70.