A Study of Reliability and Physical Properties of Schottky Barriers with Respect to Thz Applications

Whisker contacted GaAs Schottky barrier diodes are the standard devices for mixing and multiplier applications in the THz frequency range. With the decreasing size of Schottky diodes for operation at higher frequencies, the reliability and the physical understanding of the Schottky barrier becomes increasingly important. In this contribution, we present new results concerning the reliability of Schottky diodes and new insight into the physical properties of Schottky junctions, especially at low current densities. For these purposes a number of different Schottky diodes have been fabricated with varying epi-layer doping concentrations and anode diameters. It can be inferred from the measured I/V characteristics that the diode current deviates normally considerably from the ideal thermionic current behavior with decreasing diode diameter. This deviation shows an exponential dependance on the diode voltage and is a function of the doping concentration. For a given doping concentration in the epi-layer and decreasing anode diameter, this phenomenon shifts the minimum of the ideality factor towards higher current densities. It is speculated that this is caused by the crystallinity difference of the polycrystalline Pt films on the GaAs films for decreasing Si0 2 aperture size when the Pt mobility in the electrolyte of the hole is reduced. The reliability of Schottky barrier diodes under thermal and electrical stress has been investigated on different THz Schottky diode structures. The results show that the barrier height and the ideality factor of the fabricated structures is not affected by thermal stress. Electrical stress induced by large forward currents up to a current density of 10 kiljrnim 2 even leads to a slight increase of the barrier height and a reduction of the series resistance. Fourth International Symposium on Space Terahertz Technology Page 437 Introduction The physical properties of Schottky contacts on GaAs are well-known and have been a subject of investigation since the beginning of GaAs technology [1]. But this is not the case for small-area Schottky barrier junctions which are the key element for mixing applications at far infrared frequencies. Diodes for this purpose usually have the so called honeycomb design [2]with anode areas of less than 1 yrn 2 [3]. For this study a number of different diodes has been fabricated in order to investigate the influence of the diode diameter and the epi-layer doping concentration on the physical diode properties determined from the I/V characteristics. When talking about Schottky barriers, the most important parameter is the barrier height 4 ;) or the current-dependent ideality factor n(i) which describes the lowering mechanisms of the barrier. It has been already shown earlier that the current through Schottky diodes for THz applications at medium and high forward bias can be described by therrnionic emission and thermionic field emission when effects such as current spreading, heating of electrons etc are taken into account {4, 5, 6, 71. In the range of small currents (less than approx. 100 nA), a considerable deviation from the above theory can be observed, especially for small and highly doped diodes. With the decreasing diode diameter of Schottky diodes for THz mixing applications, the reliability and stability of the contacts become more important. This is mainly due to the increased current densities under mixing conditions where a typical bias current between 200 and 500 yA leads to current densities of up to 1000 A/m,m 2 . Another problem with small anode areas is the semiconductor surface technology which becomes more critical with decreasing Schottky contact area Therefore, it is necessary to know how Schottky diodes behave under extreme thermal and electrical conditions. Diode fabrication and characterization The epitaxiai layers for the diodes were all grown by the same supplier Layers with doping concentrations between 2 10 16 cm 3 and 3 10 17 cm3 have been used for the fabrication of Schottky diodes with diameters between 7 and 0.8 ,am . All diodes investigated in this study have been fabricated with the same process making use of in situ anodic pulse etching and electrolytic Pt deposition. The features of this technique have been presented earlier and are described in [1 S, 91. The only exception is the diode Dor which 'nas been exposed to air for ' Drs. H. Grothe and J. Freyer, Technical University of Munich, Germany diode diameter {1177/1 N d e de 10 17 {crn3 nrn [Q] n min Page 432 Fourth International Symposium on Space Terahertz Technology 1. 0 rran prior to the electrolytic Schottky metal deposition. Therefore. this diode is expected to have an interfacial native oxide layer of about 1 nrn thickness. The diodes were characterized by I/V and C/V measurements using a HP4151B Semzconductor Parameter Analyzer and a HP4279A C/V-Meter. respectively. Additionally, noise measurements at 1.5 GHz have been carried out using a HP897013 Noise Figure Meter. The noise temperature of the THz diodes A, C, E is shown in fig. 1. A comparison of the diode parameters is given in the following table: A 0.8 3 70 1 20 1.23 I 4.9 B 0.8 9 100 0.9 25 1.18 6.3 C 1 . 2 100 1.2 15 1.18 6.3 D 1.3 9 100 2.1 12 1.15 4.9 , Dor 1.3 2 100 2.1 14 1.25 5.5 , E 0.8 grP) 90 0.8 28 1.15 6.5 F 1.1 gr(3) 90 1.1 21 ' 1.13 6.4 G , 1.3 gr(3) 90 2.2 19 1.10 7.0 2.2 0.8 100 5.6 12 1.11 7.1 I 1.3 0.2 100 1.2 1 18 1.08 9.0 0.2 100 , 6.3 13 1.06 , 9.1 7 0.2 100 33 6 1.04 9.1 (1): minimum of the differential measured I/V characteristic (2): measured at a reverse current of -1 (3): graded doping, from 2 10 16 cm -3 at the surface to substrate doping within 90 rim Physical properties Diodes fabricated according to the above in-situ electrochemical etching and deposition process show near-ideal I/V characteristics. The measured I/V characteristics were used to characterize and to determine the physical properties of the diodes. One of the most suitable parameters for the characterization of the physical properties is the diode ideality factor n. Schottky diodes with large contact areas can be described in a wide current range b y a more or less constant 10