Low dark current far infrared detector with an optical cavity architecture

Here we report designs for performance improvements of homojunction interfacial workfunction internal photoemission (HIWIP) detectors for di€erent far infrared regions. A design is given to reduce dark current to about 10 e/s for a 300 lm cut-o€ detector at 1.3 K, at a bias ®eld of 500 V/cm by adjusting the thickness of the intrinsic layer to eliminate tunneling component. The intrinsic region thickness and the bottom contact are used to obtain an optical cavity e€ect thus increasing the absorption to almost 100%. Increased responsivity due to the optical cavity e€ect is experimentally veri®ed by using three detector structures, with three di€erent cavity structures. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Far infrared detector; Homojunction interfacial workfunction internal photoemission; Cavity e€ect High performance far infrared (FIR) (40±200 lm) semiconductor detectors as well as large focal plane arrays are required for space astronomy applications, such as NASA's airborne mission, Stratospheric Observatory for Infrared Astronomy (SOFIA), and the ESA's Far-infrared and Sub-mm Telescope (FIRST) programs. Si or GaAs homojunction interfacial workfunction internal photoemission (HIWIP) FIR detectors can compete with extrinsic Ge photoconductors (unstressed or stressed) and Ge block-impurity-band (BIB) detectors due to the material advantage of Si or GaAs over Ge [1]. The detection mechanism [1] of HIWIP detectors involves absorption in the highly-doped emitter layers mainly by free carriers followed by the internal photoemission of photoexcited carriers across the junction barrier and then collection. The cut-o€ wavelength kc is determined by the interfacial barrier height between the emitter layers and undoped intrinsic layers. Signi®cant progress has already been achieved in the development of p-GaAs HIWIP FIR detectors [2], resulting in a responsivity of 3.1 A/W and detectivity of 5:9 10 cm  Hz p /W, with kc as long as 100 lm. Experimental and calculated results were presented [3] for GaAs HIWIP detectors with relatively thin (Wi ˆ 1000ÿ 5000 A) i-regions. Typical current noise spectra demonstrated the 1=f behavior at frequencies below 1.5 kHz. It is shown that the noise power density (Si) in HIWIP depends strongly on the dark current (Id) through the detector: Si…f † ˆ CI d=AjfNis, where C 0:1, Aj is the device area, and Nis 10±10 cmÿ2 is the interface state density [4]. The HIWIP structures with the thin i-region had relatively high dark current. For example, a 1000 A thin i-region, and a 100 lm cut-o€ detector with area of 1 10ÿ3 cm, will have 10ÿ6 A dark current at 4.2 K under a ®eld of 500 V/cm [5]. Reducing the temperature did not lower the dark current indicating the dominance of tunneling. Introducing thicker i-regions to reduce Id will shift the barrier maximum position away from the interface due to the space Solid-State Electronics 45 (2001) 87±93 * Corresponding author.Fax: +1-404-651-1427. E-mail address: [email protected] (A.G.U. Perera). 1 Present address: Department of Applied Physics, Shanghai Jiao Tong University, 1954 Hua Shan Road, Shanghai 200030, People's Republic of China. 0038-1101/01/$ see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S0 03 8 -1 10 1 (00 )0 0 22 5 -2 charge (SC) e€ects [6]. Hence a tradeo€ between dark current and the quantum eciency is expected due to increased i-region thickness of the detector. This article studies the e€ect of layer thickness in reducing dark current, and increasing the absorption using an optical cavity architecture. A typical p‡±i (or n‡±i HIWIP structure consists of a heavily doped emitter layer, the intrinsic (or lightly doped) layer, the bottom contact layer, having thicknesses We, Wi , and Wb, with corresponding doping concentrations Ne, Ni, and Nb. Several units of emitterintrinsic layers could be present (for the structures studied so far), as seen in Fig. 1. For the structure with number N of i±p‡ repeats and for the thickness of the top emitter layer Wte the total thickness is WT ˆ Wbc‡ Wbi ‡ N…We ‡ Wi† ‡ Wte. It is known that the resonant cavity enhanced (RCE) devices [7] utilize the re ̄ection of the incoming radiation from the bottom mirror to increase the optical ®eld intensity in the active region of the detector, increasing the absorption leading to improved quantum eciency. It is an attractive idea to use a bottom contact layer as a ``mirror'' re ̄ecting the radiation which is not absorbed by the emitter/absorber in the ®rst pass through giving rise to a simple cavity architecture. Increasing the thickness and doping concentration of the bottom contact layer will increase the re ̄ection from it. The FIR absorption is calculated from the complex permittivity of each layer by matching the electric and magnetic ®elds at the interfaces. Far away from the reststrahlen region the frequency-dependent permittivity of the highly doped emitter layer can be considered as [8] ˆ s 1 ÿ x 2 p x…x‡ ix0† ! …1† Eq. (1) describes the free-carrier absorption in the frame of Drude model. Here x is the optical frequency, x0 ˆ 1=s is the free-carrier damping constant with relaxation time s (which is independent of frequency in semiclassical transport theory), xp ˆ …Neq= 0 sm † is the plasma frequency, and s is the low frequency dielectric constant of an intrinsic semiconductor, m ˆ 0:5m0 is the heavy-hole e€ective mass in GaAs (in one-band model), m0 is the free electron mass, and q is the magnitude of the electron charge. The carrier concentration Ne ˆ 2 10 cmÿ3 can be estimated from the doping level, while the relaxation time s 10ÿ14 s was measured for a sample with similar doping level [9]. Reststrahlen absorption can be taken into account by modifying Eq. (1) with speci®c term [8]. The substrate and the i-regions were considered transparent faraway from the reststrahlen region with permittivity s. The permittivity of the bottom contact layer can be calculated by Eq. (1) with using the appropriate concentration Nb. By using the above expressions, absorption, re ̄ection, and transmission coecients for GaAs p‡-i structures have been calculated. As expected, the absorption maxima for the structure with one emitter are realized at wavelengths corresponding to ks…1‡ 2n†=4 distance from the bottom contact layer where ks is the wavelength inside the media, with n ˆ 0; 1; 2; . . .. Optical ®eld distribution inside the structure for a normal incident electro-magnetic wave with wavelength corresponding to the ®rst absorption maximum (n ˆ 0), is shown in Fig. 2. It is seen that the absorption maximum is reached Fig. 1. The schematics of the tested multilayer HIWIP detector. p‡‡ is the contact layer, p‡ the emitter layer, and i the undoped layer. A window is opened on the top side for frontside illumination. The projection shows the N-multilayers, replaced by a thicker single iregion. 88 A.L. Korotkov et al. / Solid-State Electronics 45 (2001) 87±93