Heterojunction Internal Photoemission Research Articles

Study on the long wavelength SiGe/Si heterojunction internal photoemission infrared photodetectors

The theory of internal photoemission in semiconductor heterojunctions has been investigated and the existing model has been extended by incorporating the effect of different effective masses in the active region and the substrate, nonspherical-nonparabolic bands, and the energy loss per collision. Photoresponse measurements on Si 1− x Ge x /Si heterojunction internal photoemission (HIP) infrared photodetectors (IP) have shown that they are fit well by the theory. Qualitative model describing the mechanisms of photocurrent generation in our structures are presented. We also study the effect of a double barrier on the photoresponse spectrum of a SiGe/Si HIP IP. It has been shown that the performance of our devices depends significantly on the applied bias and the operating temperature; therefore, their cut-off wavelengths can be tuned to the desired region by changing the potential difference across the device and/or changing the device temperature. The barrier heights (correspondingly the cut-off wavelengths) of the samples have been determined from their IP spectra by using the extended model which has the wavelength and doping concentration dependent free carrier absorption parameters.

Mechanisms of photocurrent generation in SiGe/Si HIP infrared photodetectors

We study the mechanisms of photocurrent generation in the SiGe/Si heterojunction internal photoemission (HIP) infrared photodetectors and present a qualitative model. It has been shown that the performance of our devices depends significantly on the applied bias and the operating temperature. The device presented here allows us to tune the cut-off wavelength between 28.3 µm and 17.8 µm by changing the potential difference across the device.

Double-barrier long wavelength SiGe/Si heterojunction internal photoemission infrared photodetectors

This work studies the effect of a double barrier on the photoresponse spectrum of SiGe/Si heterojunction internal photoemission infrared photodetectors. Results are compared with those of a single barrier sample which has nominally the same parameters. It is shown that the cut-off wavelength of the multi-barrier device depends on the operating temperature; therefore, it can be tuned to the desired region within the device capacity by changing the device temperature. The proof of concept device, which has two Si1-xGex/Si junctions with x=0.1 and x=0.23, allows the tuning of the cut-off wavelength from 19.9 μm to 11.7 μm for 10 K and 50 K, respectively.

Structure and doping optimization of SiGe heterojunction internal photoemission detectors for mid-infrared applications

We present full-scale calculations of highly p-type doped siGe heterojunction internal photoemission (Hip) detectors that operate in the mid-infrared (3- to 5-?m) range of wavelength. We explore the effects of including undoped spacer layers within the highly doped siGe wells, for which a systematic body of experimental data is available to us, and which demonstrates a substantial reduction in the dark current produced by these devices. We model the doping explicitly by means of a screened Coulomb potential, leading to a full description of the resultant impurity band, and compare our calculated optical line shapes with recent state-of-the-art molecular-beam epitaxy experiments. The variation of the optical response with well width, germanium concentration, spacer width and position, and doping concentration are all considered.

Direct optical measurement of the valence band offset of p+ Si1−x−yGexCy/p− Si(100) by heterojunction internal photoemission

Optical absorption measurements have been performed to study the effect of substitutional carbon on the valence band offset of compressively strained p+ Si1−x−yGexCy/(100) p− Si. The compressively strained p+ Si1−x−yGexCy/(100) p− Si heterojunction internal photoemission structures were grown by rapid thermal chemical vapor deposition with substitutional carbon levels up to 2.5%. Carbon decreased the valence band offset by 26±1 meV/% substitutional carbon. Based on previous reports of the effect of carbon on the band gap of Si1−x−yGexCy, our work suggests that the effect of carbon incorporation on the band alignment of compressively strained Si1−x−yGexCy/Si is to reduce the valence band offset, with a negligible effect on the conduction band alignment.

Improvement on the performance of P +-Ge xSi 1− x/p-Si heterojunction internal photoemission infrared detectors at 77 K

By using a stacked structure, adding an SiO 2 dielectric cavity and an Al mirror, and evaporating a SiO anti-reflecting layer on the back of the substrate, the peak detectivity D ∗ (5.5,1000,1) of an unbiased P +-Ge x Si 1− x /p-Si heterojunction internal photoemission (HIP) infrared detector at 77 K has been improved to 1.1×10 10 cm Hz 1/2/W, with a peak quantum efficiency of ∼2.8% and a photoresponse range of 2–8 μm. Its D ∗ p is of the same order of magnitude as that of the PtSi infrared detectors being used in practice. In addition, an improved electrode structure is used in order to improve the performance and reliability of the device.

Direct Optical Measurement of the valence band offset of p+ Si1−x−yGexCy/p− Si (100) by Heterojunction Internal Photoemission

AbstractOptical absorption measurements have been performed to study the effect of carbon on the valence band offset of compressively strained p+ Si1−x−yGexCy/(100) p− Si heterojunction internal photoemission structures grown by Rapid Thermal Chemical Vapor Deposition (RTCVD) with substitutional carbon levels up to 2.5%. Results indicated that carbon decreased the valence band offset by 26 ± 1 meV/ %C. Results from optical measurement in this study agreed with previous data from capacitance-voltage measurements. Based on previous reports of carbon effect on the bandgap of compressively strained Si1−x−yGexCy, our work suggests that the effect of carbon incorporation on the band alignment of Si1−x−yGexCy/Si is to reduce the valence band offset, with a negligible effect on the conduction band alignment.

Ge x Si 1−x infrared detectors I. Absorption in multiple quantum well and heterojunction internal photoemission structures

We report absorption measurements on two types of long-wave infrared detector structures. Both types were grown by ultrahigh vacuum chemical vapor deposition, and were characterized by multiple analytic techniques. In both multiple quantum well (MQW) and heterojunction internal photoemission (HIP) structures, it is found that free-carrier absorption is dominant for normally incident radiation. The measured absorption is fit well by the classical expression for free-carrier absorption, with scattering times of about 10−14 s (MQW) and 5×10−15 s (HIP). The measured absorption is used to evaluate the responsivity that results when all carriers energetically able to surmount the barrier are collected. Based on this analysis, higher responsivity is predicted for HIP detectors, largely because of the greater density of initial states. The responsivity obtained in practice depends upon the photoconductive gain (MQW detectors) or the escape probability (HIP detectors). The escape probability for HIP detectors is measured in Part II.

Ge x Si 1−x infrared detectors II. Carrier escape probability and detector performance

Ge x Si 1−x / Si heterojunction internal photoemission (HIP) detectors with thresholds in the medium-wave infrared and long-wave infrared (LWIR) regions were fabricated and characterized. Measurements of the photoresponse are fit well by a theory which takes into account the scattering of excited carriers. The probability of escape of an excited hole is calculated and compared with that observed in another detector, the multiple quantum well structure. It is shown that HIP detectors can achieve background-limited performance in the LWIR region when operated at 40 K.

GeSi infrared detectors

Germanium–silicon heterojunctions can be used to fabricate detectors suitable for use in far-infrared focal plane arrays. We report here the growth and characterization of multiple quantum well and heterojunction internal photoemission (HIP) structures which have thresholds suitable for the 8–12 μm region. We have used absorption measurements to show that free-carrier absorption is dominant in both structures for normally incident illumination. The HIP structure is more attractive from the point of view of integrability with silicon readout circuitry. We will report the results of optical and electrical characterization of HIP detectors which show that this detector can achieve background-limited infrared performance at an operating temperature of 40 K.

Infrared-sensitive SiGe–Si heterojunction internal photoemission detectors produced by rapid thermal chemical vapour deposition

We report the growth of Si 1− X Ge X on Si by rapid thermal chemical vapour deposition (RTCVD) for heterojunction internal photoemission (HIP) photodiodes. The emitter is a heavily in situ boron-doped SiGe layer with a Ge content of up to 46%. Photodiodes are characterized by Richardson plot analysis and Fourier transform infrared spectroscopy. A strained layer containing 30% Ge results in a device working at temperatures of up to 80K, with a cut-off wavelength of 8.5 μm. First results with a polycrystalline SiGe emitter layer are also presented, opening the possibility of fabricating stacked optoelectronic devices.

Optical and electrical properties of Si 1− x−yGe xC y thin films and devices

Si 1− x−y Ge x C y thin films grown by low temperature chemical vapor deposition containing up to 1.2% substitutional C were incorporated into microelectronic devices. The chief aims are to demonstrate that the material is of suitable quality for device applications and to use the electrical and optical characteristics of the devices to study the material. Three sets of devices containing epitaxial Si 1− x−y Ge x C y layers were fabricated and characterized. The temperature dependence of the collector current in Si 1− x−y Ge x C y base heterojunction bipolar transistors indicated that the band gap of Si 1− x−y Ge x C y increases +26 meV %C −1. Capacitance measurements on p +-Si 1− x−y Ge x C y /p −-Si heterojunction internal photoemission structures indicated that the increase in band gap in the Si 1− x−y Ge x C y is due primarily to downward movement of the valence band. Finally, Si 1− x−y Ge x C y p–i–n diodes showed no degradation in reverse bias leakage compared with C-free devices for y<0.01. However, the diodes did exhibit excess sub-band gap absorption which increased with [C].

A Novel Infrared Sige/Si Heterojunction Detector with an Ultrathin Phosphorus Barrier Grown by Atomic Layer Deposition

ABSTRACTIn strained Sia−xGex heterojunction internal photoemission (HIP) photodiodes the spectral response can be tailored over a wide wavelength range by varying the Ge content. In this paper heavily in situ boron doped SiGe layers with 30% Ge were grown by low pressure rapid thermal chemical vapour deposition (LP(RT)CVD). The detectors exhibit a cut-off wavelength of 8.5 μm. A delta-like P peak, incorporated by atomic layer deposition technique, shifts the cut-off to shorter wavelengths. This shift is related to an increase of the barrier height at the SiGe/Si interface caused by the narrow n+-doped layer in agreement with device simulation. In this way the trade off between critical film thickness for high Ge content film growth and absorption depth for proper detector response can be overcome.

Growth of epitaxial GexSi1 − x for infrared detectors by UHV/CVD

We report the application of UHV/CVD (ultra-high vacuum chemical vapor deposition) to the growth of heterojunction internal photoemission (HIP) infrared detector structures. The HIP structure is essentially a Schottky barrier junction formed between a degenerately doped germanium-silicon absorbing layer and an undoped or lightly doped silicon layer. This device places stringent demands on the background doping concentration and morphology of the epitaxial layers. C(V) profiling shows that acceptably low doping of the silicon layer is obtained only after a cleanup run. Surface morphology of the Ge x Si 1 − x absorbing layer is degraded by the formation of thickness undulations, especially at the large boron concentrations required. By appropriate changes to the growth procedure, we have been able to fabricate working detectors with good electrical characteristics.

ReSi2 thin-film infrared detectors

Two types of thin-film infrared-sensing devices have been investigated using the narrow band-gap semiconductor, rhenium disilicide (Eg∼0.1 eV). These are the ReSi2/n-Si heterojunction internal photoemission (HIP) detector and the ReSi2 thin-film photoconductor. The HIP device was found to be rectifying and to obey a Fowler-type law with a long-wavelength cutoff of ∼2.1 μm (0.59 eV) at room temperature. In hopes of approaching the fundamental limit for a ReSi2-based photonic detector, ∼12 μm (0.1 eV), the ReSi2 photoconductor was explored. Indeed, the spectral response (measured at 10 K) of the ohmic photoconductor was found to extend to 6 μm (the present limit of our measurement equipment), with no indication of a detection cutoff.

MBE Growth Considerations for the Fabrication of 640×480 IR Focal Plane Arrays of SiGe Hip Detectors

AbstractEncouraging results have been reported for discrete heterojunction internal photoemission (HIP) infrared (IR) detectors composed of heavily boron doped Si1−3Gex layers on Si. We desired to build on those results and fabricate 640×480 IR focal plane arrays on 100 mm Si substrates, suitable for commercial microelectronic processing. In this paper we discuss the growth issues for growing these structures by molecular beam epitaxy. Since the wafers had already undergone processing and some had PtSi contacts, the growth temperature was constrained to be no greater than 600 °C. Precise temperature control was obtained by calibrating an optical pyrometer with a thermocouple embedded in the substrate heater assembly, which was calibrated using the eutectic emperatures of Au/Si and Al/Si. The final step of the cleaning process was a 1% HF dip/ spin dry, which resulted in a H-terminated surface. The H was removed at 550 °C in vacuum prior to rowth. The growth of the B-doped SiGe layer was done at 350 °C to minimize segregation and diffusion of the Ge and B. Doping levels of 2×1020/cm3 were obtained with near 100% activation. Using Si0.35, doped with 2×1020 B/cm3, a cut-off wavelength of 11.1 μm and an emission coefficient of 19.8 %/eV were obtained for discrete detectors. Preliminary results from the detector arrays show full functionality in the spectral range of 6.1 to 12.8 μm.

GexSi1−x Heterojunction Internal Photoemission Structures by Ultra High Vacuum Chemical Vapor Deposition

ABSTRACTGexSi1−x, heterojunction internal photoemission devices for detection of infrared radiation have been fabricated from films grown by ultra high vacuum chemical vapor deposition (UHV/CVD). Films and devices were characterized by Fourier transform infrared spectroscopy (FTIR), photoresponse, and electrical measurements. Detector efficiencies were found to increase substantially (from ∼ 1.5 to ∼ 20% per eV) as the boron concentration in the absorbing layer was increased from 1×1020 to 2.5×1020 cm−3. Increases in boron concentration beyond 2.5×1020 cm−3 did not significantly improve device performance. This result is attributed to incomplete dopant activation at higher boron concentrations.

Conduction and valence band photoemission mechanisms in two-dimensional–three-dimensional structures

The active layers in heterojunction internal photoemission infrared detectors are always of thickness in the order of few hundred Å, and thus the behavior of the excited carriers should be described in a two-dimensional (2D) mode. Conduction and valence band photoemission mechanisms have been introduced in 2D–three-dimensional structures. The carrier behaviors of two carrier types, electron and hole, were discussed in the cases of PtSi/p-Si and p+-SiGe/p-Si, respectively. Parallel momentum conservation were well preserved for these two cases during carrier transportation across the heterojunction ballistically. Theoretical simulations were in good agreement with experimental data for both the electron and hole cases. The reason why p+-SiGe/p-Si photodiodes exhibit higher quantum efficiency than PtSi/p-Si diodes has also been discussed.

Long-wavelength stacked SiGe/Si heterojunction internal photoemission infrared detectors using multiple SiGe/Si layers

Utilizing low temperature silicon molecular beam epitaxy growth, long-wavelength stacked SiGe/Si heterojunction internal photoemission (HIP) infrared detectors with multiple SiGe/Si layers have been fabricated and demonstrated. Using an elemental boron source, high doping concentration (≊4×1020 cm−3) has been achieved and high crystalline quality multiple Si0.7Ge0.3/Si layers have been obtained. The detector structure consists of several periods of degenerately boron doped (≊4×1020 cm−3) thin (≤50 Å) Si0.7Ge0.3 layers and undoped thick (≊300 Å) Si layers. The multiple p+-Si 0.7Ge0.3/undoped-Si layers show strong infrared absorption in the long-wavelength regime mainly through free-carrier absorption. The stacked Si0.7Ge0.3/Si HIP detectors with p=4×1020 cm−3 exhibit strong photoresponse at wavelengths ranging 2–20 μm with quantum efficiencies of about 4% and 1.5% at 10 and 15 μm wavelengths, respectively. The detectors show near ideal thermionic-emission limited dark current characteristics.

Photoresponse model for Si/sub 1/spl minus/x/Ge/sub x//Si heterojunction internal photoemission infrared detector

A photoresponse model has been developed for the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> spl minus/x/Ge/sub x//Si heterojunction internal photoemission (HIP) infrared detector at wavelengths corresponding to photon energies less than the Fermi energy. A Si/sub 0.7/Ge/sub 0.3//Si HIP detector with a cutoff wavelength of 23 μm and an emission coefficient of 0.4 eV/sup /spl minus/1/ has been demonstrated. The model agrees with the measured detector response at /spl lambda/>8 μm. The potential barrier determined by the model is in close agreement (difference /spl sim/4 meV) with the potential barrier determined by the Richardson plot, compared to the discrepancies of 20-50 meV usually observed for PtSi Schottky detectors.