Journal of Spectroscopy

Volume 2019, Article ID 1915320, 23 pages

https://doi.org/10.1155/2019/1915320

## Free-Electron Screening Mechanism of the Shallow Impurity Breakdown in n-GaAs: Evidences from the Photoelectric Zeeman and Cyclotron Resonance Spectroscopies

^{1}Institute of Physics, Azerbaijan National Academy of Sciences, Huseyn Javid Avenue 33, AZ-1143 Baku, Azerbaijan^{2}Faculty of Physics, Moscow State University, Baku Branch, Str. Universitetskaya 1, AZ-1144 Baku, Azerbaijan

Correspondence should be addressed to E. P. Nakhmedov; za.usm@vodemhkan.revne

Received 27 December 2018; Accepted 6 May 2019; Published 30 May 2019

Academic Editor: Jau-Wern Chiou

Copyright © 2019 O. Z. Alekperov and E. P. Nakhmedov. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A novel breakdown (BD) mechanism of shallow impurity (SI) under the electric field at low temperatures is suggested for samples with the donor concentrations and the compensation degree with acceptors of concentration in the external magnetic fields up to , oriented parallel or perpendicular to the external electric field. Diagnosis of the BD mechanism was performed by SI Zeeman (mainly from the ground state to and other excitation states) and cyclotron resonance photoelectric spectroscopy (PES) methods in the wide interval of the electric field including the BD region too. The obtained results reveal that the BD electric field does not correlate with *K* and the carrier’s mobility *μ* of the samples, which contradict to the well-known impact ionization mechanism (IIM). A serious discrepancy with IIM is that does not almost depend on the magnetic field up to when though the SI ionization energy increases two times. The cyclotron resonance (CR) measurements show that the line width does not depend on the electric field for , indicating the lack of free-carrier (FC) heating in contradiction with IIM. A considerable decrease of the free carriers’ capture cross section (CCS) area by ionized SI centers with a subsequent increase in the FC concentration *n* is observed by means of PES investigation of the and CR lines in the electric fields and at different magnetic fields, applied along () the electric field or perpendicular () to the electric field. The slope of the line intensity on the electric field for does not depend on the magnetic field, which is valid for too. Various effects determined in the PES measurements at , such as a drastic narrowing of the and CR lines, a shift of the CR line to higher magnetic fields, and disappearing of the lines to higher excited SI states, were clarified to be a result of screening of the SI Coulomb potential by free carriers. The FC screening at the BD reduces the potential fluctuation and its influence to the PES line shape of and other excited states. It is shown that an increase in the FC concentration reduces the CCS, which can be assumed as the main factor along with the increase in the ionization coefficient for the SI breakdown in the electric field. The screening length of the SI Coulomb potential decreases with the increasing FC concentration, reducing the CCS; the latter seems to vanish completely at ( is the effective Bohr radius), when high screening results in vanishing of all the bound states of the Coulomb potential. Note that this limit is similar to the Mott transition. Many experimental facts and our calculation of the CCS support the suggested mechanism for the SI breakdown. The well-known IIM is valid for samples with SI concentrations and takes place at very high electric fields.

#### 1. Introduction

The gallium arsenide is one of the most utilized semiconductors in the modern electronics technology. epitaxial layers are the widely used heterojunctions for the investigation of the comprehensive class of 2D modern electronic and spintronic phenomena and for fabrication of different devices on their basis such as Gann diodes, photodetectors operating in the wide range of frequency, and high-frequency field transistors. [1–6]. In order to improve the electrophysical characteristics of these devices, fabricated by using high-purity semiconductors, it is necessary to know the chemical nature and the relative concentration of residual impurities. Many of these devices operate at low temperatures and sufficiently high electric fields when shallow impurities (SI) breakdown (BD) takes place (see, for review, e.g., [7, 8]). Therefore, more careful investigation of the SIBD mechanism in is essential.

The low-temperature submillimeter wave photoelectric spectroscopy (PES) of SI is the most sensitive method for identification of the SI contents in semiconductors [9]. Significant information on SI can be obtained from the photoconductivity spectral line width and the line broadening mechanism. The line width of the SI PES was shown [10, 11] to be determined by the concentrations of the major and compensated impurities as well as their distribution. The difference of ground state energies for different SI atoms, which is called a chemical shift (CS) or central cell correction, is a result of deviation () of SI Coulomb potential at small distances from pure Coulomb form. The value of the CS for SI in can be estimated to be , where and are the unit cell size and the effective Bohr radius, respectively, and is the effective Rydberg. This means that the energies of all SI in lay in the interval of ; therefore, CS correction to higher excited states of SI ( and , …) is much smaller than that for state and can be neglected. For the purest semiconductors with donor concentrations of , the photoexcitation lines of impurities, the intensity of which is proportional to the concentration of the corresponding impurity, are broadened due to the different values of the CS, and the broadening value for samples is the same order as the energy distance between impurities. Each impurity atom is considered in this case to be isolated, and the line width broadening is determined only by the charged impurity mechanism. As we will show in the following section, the quadratic Stark effect and the quadrupole-gradient shifting of neutral impurities’ levels are responsible for the inhomogeneous broadening of the charged impurities. Note that such a small value of CS, which is the same order as the impurity photoexcitation line width, is inherent to the most of semiconductors (, , and ). However, the difference of SI ionization energies for different impurities in and is in the order of . That is why the CS in these materials does not affect the line width.

For samples with the SI concentrations higher than , the picture is significantly different [12]. First of all, our experiments have determined that the line width for these samples does not correlate with the SI concentration, so that for a sample with smaller concentration, the line width may be larger than that of more doped samples (Table 1). Our investigations show that the line width of these samples is determined not only with the SI concentration but with the potential fluctuation due to the inhomogeneous SI donors and acceptors distribution too. Such an inhomogeneous SI distribution creates a potential fluctuation, which gives an additional contribution to the line width. At lower temperatures, close to zero, the electrons are captured by impurities, providing a correlated distribution of electrons, which is realized by minimizing the total Coulomb energy of the system of the charged donors, acceptors, and electrons [13]. At higher temperatures, when with being the mean distance between charged impurities, the electrons are activated, and their distribution becomes random. The transition line width in this case depends on the charged impurity concentration and does not depend on the compensation . Instead, the line width in the real experiments, which are realized at lower temperatures, the electrons distribution is correlated, since the acceptors are charged, taking an electron from the nearest donor [11], and the former strongly depends on *K*. A transformation from the correlated to the random distribution can be reached not only by increasing temperature but also by applying an external electric field.