V. Garber

Gain mechanism in GaN Schottky ultraviolet detectors

Schottky barrier GaN ultraviolet detectors, both in vertical and in lateral configuration, as well as in a metal–semiconductor–metal geometry were implemented. All devices exhibit a high gain at both reverse and forward bias. The photoresponse in the forward bias is in the positive current direction. We attribute the gain to trapping of minority carriers at the semiconductor–metal interface. The excellent agreement between the calculated responsivity and the experiment indicates that the model is valid for all device structures under study, and represents a unified description of gain mechanism in GaN Schottky detectors.

On the extraction of linear and nonlinear physical parameters in nonideal diodes

We describe a parameter extraction technique for the simultaneous determination of physical parameters in nonideal Schottky barrier, p-n and p-i-n diodes. These include the ideality factor, saturation current, barrier height, and linear or nonlinear series, and parallel leakage resistances. The suggested technique which deals with the extraction of bias independent parameters makes use of the forward biased current–voltage (I–V) characteristics and the voltage-dependent differential slope curve α(V)=[d(ln I)]/[d(ln V)]. The method allows (a) establishment of the current flow mechanisms at low and high bias levels, (b) extensive of the permissible ranges of determined parameters beyond what is possible in other published methods, and (c) to automation and computerization of the measurement processes. The method is verified experimentally using metal–semiconductor structures based on Si, InGaP, and HgCdTe as well as an InGaAs/InGaAsP multiple quantum well laser diode exemplifying a p-n junction.

Anisotropy in detectivity of GaN Schottky ultraviolet detectors: Comparing lateral and vertical geometry

Vertical and lateral geometry GaN-based Schottky barrier photodetectors have been implemented, using similar quality material and the same fabrication process. The vertical detector exhibits two orders of magnitude higher responsivity. This is attributed to improved ohmic backcontacts, due to the highly doped buried layer. The vertical detectors exhibits also lower 1/f noise level, which is attributed to the reduced effect of dislocations on the carrier transport, resulting in lower mobility fluctuations. The vertical detector normalized detectivity is four orders of magnitude higher.

Electrical properties of epitaxially grown CdTe passivation for long‐wavelength HgCdTe photodiodes

Results of experimental measurements and theoretical analysis are presented for the TiAu/ZnS/CdTe/HgCdTe metal–insulator–semiconductor heterostructure. The passivation of HgCdTe is provided by a double layer consisting of a dielectric ZnS placed on top of an epitaxial CdTe layer. Both HgCdTe and CdTe were grown by metalorganic chemical vapor deposition. Two types of CdTe layers were investigated: one was grown directly, in situ, immediately following the growth of HgCdTe; the second was grown indirectly using previously grown HgCdTe samples. It is shown that directly grown CdTe layers lead to low fixed interface charge, which is a good condition for passivation. The indirectly grown samples are still acceptable, but not as good as the directly grown samples. We demonstrate, on the basis of theoretical considerations, that the dielectric ZnS improves the flatband condition at the CdTe/HgCdTe interface.

Electrical and structural properties of epitaxial CdTe/HgCdTe interfaces

In this study, CdTe epilayers were grown by metalorganic chemical vapor deposition on epitaxial HgCdTe with the purpose of developing suitable passivation for HgCdTe photodiodes. Two types of CdTe layers were investigated. One was grown directly,in situ, immediately following the growth of HgCdTe. The second type of CdTe was grown indirectly, on top of previously grown epitaxial HgCdTe samples. In this case, the surface of the HgCdTe was exposed to ambient atmosphere, and a surface cleaning procedure was applied. The material and structural properties of the CdTe/HgCdTe interfaces were investigated using secondary ion mass spectroscopy, Auger electron spectroscopy, Rutherford back scattering, and x-ray double crystal diffractometry techniques. Electrical properties of the CdTe/HgCdTe heterostructure were determined by capacitance-voltage (C-V) characterization of Schottky barrier devices and metal insulator semiconductor devices. Also, a preliminary current-voltage characterization of n+ p photodiodes was performed. A theoretical model suitable for analysis of graded heterojunction devices was used for interpretation of C-V measurements.

Estimation of HgCdTe band‐gap variations by differentiation of the absorption coefficient

A new simple nondestructive method for band‐gap estimation in HgCdTe layers is presented in this work. The theoretical approach is based on a qualitative empirical model that assumes two different regions of absorption in HgCdTe. The border between the two regions, which indicates the approximate value of the band gap, is determined by differentiation of the absorption coefficient in respect to the photon energy. The approach is verified by experimental measurements on several HgCdTe layers grown by different techniques. First, the transmission is measured by Fourier transform infrared (FTIR) spectroscopy at room temperature; then, the measured data is smoothed and the absorption coefficient extracted. The absorption coefficient is then differentiated twice in respect to the photon energy which allows estimation of the average band gap and band‐gap variations within the HgCdTe layer. It appears that this simple procedure can assist in monitoring the quality of HgCdTe layers and help predicting the cutoff ...

Monitoring HgCdTe layer uniformity by the differential absorption technique

In this letter, we demonstrate how the differential absorption technique can be applied to study band gap uniformity in HgCdTe samples. The transmission of HgCdTe wafers is measured at room temperature and data filtering is used to remove both the interference fringes and high‐frequency noise. This data treatment approach produces reliable transmission data for HgCdTe layers as thin as 8 μm. In addition, the spectrum of the interference fringes is used to estimate the HgCdTe layer thickness with an accuracy of ±0.1 μm. The absorption coefficient is differentiated twice with respect to the photon energy and an approximate value of the band gap is estimated from extrema of the derivatives. By applying this procedure to different points on the same HgCdTe wafer, we can determine both lateral and transverse fluctuations of the semiconductor band gap. The initial results indicate that the accuracy of the differential absorption technique is around ±0.5 meV for HgCdTe wafers. The differential absorption techniq...

THE EFFECTS OF BUILT-IN ELECTRIC FIELD ON THE PERFORMANCE OF COMPOSITIONALLY GRADED P-ON-N HGCDTE HETEROJUNCTION PHOTODIODES

The results of a study of the optical and electrical performance of P‐on‐n HgCdTe heterojunctions, with a linear gradient in composition, are presented. First presented is a solution of the one‐dimensional continuity equation which includes the spatial dependencies of the material properties, such as absorption coefficient, band gap, and intrinsic carrier concentration, in the graded layer. Using the above solution, diodes with different grading profiles (and hence different electric fields), but with the same cutoff wavelength, are compared. The advantages of the built‐in electric field formed by the grading in composition are presented and discussed. It is shown that the major consequence of the electric field is the reduction of the effects caused by the imperfect CdTe/HgCdTe interface at the backside.

Compositionally graded HgCdTe photodiodes: prediction of spectral response from transmission spectrum and the impact of grading

We have studied the infrared transmission spectrum and the optical performance of HgCdTe photodiodes containing a linear composition gradient in the active layer. Our objectives were to enable the prediction of the optical performance of a photodiode at 77K, based on the easily and nondestructively measured transmission spectra, as well as to gain a better understanding of the effects of the grading on the optical performance. Consequently, we address three issues here. We first establish improved characterization techniques that can provide accurate values of the necessary material parameters such as gradient in composition. Second, we present a model that can predict the optical response of a diode, based on the material properties and the diode’s geometry. Third, we use the above-mentioned model for the theoretical calculations of the effects of the grading and the resulting built-in electric field on the diode’s optical response.

QUANTUM EFFICIENCY AND SPECTRAL RESPONSE OF COMPOSITIONALLY GRADED HGCDTE P-N HETEROJUNCTION PHOTODIODES

Results are presented of a study of the quantum efficiency of HgCdTe heterojunction photodiodes. All heterojunctions considered in the study consist of a wide‐band‐gap P‐type layer and a narrow‐band‐gap n‐type layer, and are illuminated from the backside. The n‐type layer is compositionally graded and therefore contains a built‐in electric field. Due to the difference in band gaps photons are absorbed in the active n‐type layer only and are collected by both drift and diffusion mechanisms. The one‐dimensional continuity equation is first solved in the linearly graded n‐type region under illumination conditions, and then the dependence of the quantum efficiency on the resulting built‐in electric field is presented. Two different modes of illumination are compared: In the first mode, associated with n‐on‐P HgCdTe diodes, the light reaches the transparent P layer first; in the second mode, associated with P‐on‐n diodes, the light reaches the transparent P side after it passes through the opaque n‐type layer....