Crystal structure of germanium

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In this chapter, the apparatus and techniques used will be described. Section 3.2 presents sample preparation, i.e. cleaning process, ohmic and Schottky contact fabrication. In section 3.3 Schottky diode characterization techniques (i.e. current-voltage (I-V), capacitance-voltage (C-V), and temperature dependent I-V and C-V are discussed. Deep level transient spectroscopy (DLTS), a defect characterization technique is outlined in section 3.4. Section 3.5 deals with the Rutherford backscattering spectrometry, utilised to study the in-diffusion of thin film metals on germanium. Scanning electron microscope system is presented in section 3. 6. This chapter ends with a discussion of annealing apparatus in section 3.7.

Sample preparations

Prior to metallization, the germanium samples were cut into (0.5 × 0.3) cm2 followed by a chemical cleaning process. Cleanliness is essential to achieving high yields and reproducible process in the production of any microelectronic devices [1]. Before metallization the semiconductor should be atomically as clean as possible and stoichiometrically perfect [2], in order to obtain high quality Schottky barrier diodes with a low leakage current when reverse biased.

Germanium cleaning process

Before metallization the following cleaning procedure was followed:

  • Degreasing the samples in trichloroethylene (TCE), acetone and then methanol, each for five minutes in an ultrasonic bath at room temperature. The degreasing process is followed by rinsing the sample in de-ionized water of resistivity < 18 M cm.
  • Etching in a solution of 30% H2O2:H2O (1:5) for one minute for the removal of the remaining oxide layer and insoluble organic contaminants and then followed by rinsing in de-ionized water.
  • The samples are blown-dry in a stream of filtered, dry, nitrogen gas, and ready for metallization.

Ohmic and Schottky contact fabrication

The ohmic contacts were fabricated by resistively evaporating a 100 nm thick layer of Au-Sb (0.6% Sb) on the back surface of the germanium samples. The deposition follows the evacuating of the deposition chamber to pressure below 10-6 Torr. The samples were then annealed for ten minutes at 350°C in Ar, to optimize the ohmic contact by lowering the barrier height, hence reducing its resistivity [3]. The annealing process results in the infusion of Sb into the backside of the germanium resulting in the increase of doping concentration. This leads to a very thin depletion region, known as a tunnel junction [4], through which field emission can take place. The use of Ar gas reduces oxidation of the metal during the annealing process, while it can also reduce interfacial oxide between the semiconductor and the metal.
A schematic of the vacuum resistive deposition system is shown in Fig. 3.1. A current flows through the crucible containing the metal to be evaporated. The molten metal evaporates and deposits onto the sample mounted above the crucible. Only metals with melting points less than 1 600°C are evaporated from this system.
Before Schottky contact deposition, the samples were again chemically cleaned as described in section 3.2.1. The Schottky contacts 0.6 mm in diameter, 30 nm thick were either fabricated by resistive deposition or electron beam deposition.
The electron beam deposition (EBD) system is depicted in Fig. 3.2. In the electron beam deposition system, a hot filament emits a beam of electrons, focused onto the crucible containing the metal by magnetic and electric fields, resulting in the melting and evaporation of the metal which deposits onto the sample. Fig. 3.3 shows the sample after the metallization process.

Current-Voltage and Capacitance-Voltage measurements

After contact fabrication, electrical characterization through current-voltage (I-V) and capacitance-voltage (C-V) measurements were carried out at room temperature. The (I-V) measurements were made by an HP 4140 B pA meter/ DC voltage source, capable of measuring current limit of 10-14 A. The (C-V) characteristics were measured by an HP 4192A LF Impedance Analyzer. The samples were measured in the dark and screened from electrical noise by enclosing them in a light-tight metal box. The most important diode parameters obtained from the (I-V) and (C-V) measurements are series resistance Rs, the barrier height fB( I V ) , the ideality factor n , reverse current Ir measured at a reverse bias of 1 V and free carrier density N D . Fig. 3.4 depicts a schematic diagram of the (I-V), (C-V) station used during the electrical characterization of the samples. Isochronal annealing of the samples was performed in the temperature range 25oC to 700oC in steps of 25oC for 30 minutes in Ar atmosphere. I-V and C-V measurements followed each annealing cycle.
Temperature dependent (I-V) measurements, in the 20-300 K temperature range were performed in a He cryostat using an automated Labview program. From the measurements, ideality factor (n), series resistance (Rs), reverse saturation current (Ir), and barrier height ( F B( I V ) ) as function of temperature, were extracted. Temperature dependent (C-V) measurements were recorded at a frequency of 1 MHz for voltage ranging from 0 V to –1 V and temperature range of 20-300 K via a Labview program. The barrier height ( F B( I V ) ) and the carrier concentration (ND) were obtained from the plot of 1/C2 vs V.

Deep level transient spectroscopy (DLTS) and Laplace-DLTS system

The layout of the DLTS and LDLTS system used in this study is illustrated in Fig. 3.5. The system consists of:

  • A cryostat, in which the sample is mounted. The cryostat temperature is controlled by a Lakeshore 340 temperature controller in the range 16-380 K.
  • Boonton 7200 capacitance meter, with 100 mV, 1 MHz AC ripple voltage, and it monitors thermal emission after excitation by a pulse generator
  • A Laplace card, which has an internal pulse generator for generating the appropriate quiescent bias and pulses. This card is also the data collection and processing system which analyses and averages the transients prior to displaying the spectra for either the conventional DLTS or L-DLTS.
  • HP 33120 15 MHz Function waveform external generator, for providing the desired quiescent bias and filling pulse to the diode which are not provided by the Laplace card.

The Laplace card sets the sample excitation parameters, capacitance transient acquisition conditions, and then initiates the measurement, acquiring the transient and converts it into either a DLTS or LDLTS spectrum depending on the measurements. For conventional DLTS the capacitance transients after excitation are measured by the capacitance meter. These transients are then processed by the Laplace card. As the temperature is ramped a DLTS spectrum is displayed on a computer for a particular rate window. L-DLTS gives an intensity output as a function of emission rate. The measurement is carried out at a fixed temperature, and several capacitance transients are captured and averaged. In the isothermal DLTS method, the sample temperature is held constant while the sampling time is varied. This technique also uses the equation

Rutherford backscattering spectrometry

Rutherford backscattering spectrometry (RBS) is a technique based on the analysis of the energy of the backscattered charged particles such as He+ used in this study. In this study, RBS technique was used to investigate the in-diffusion of metals deposited on Ge at different annealing temperatures. The schematic diagram of the RBS system attached to a Van de Graaff 2.5 MeV at the University of Pretoria is depicted in Fig. 3.6. The charged particles are generated from a gas introduced into the ion source, and accelerated to high energies up to 1.8 MeV by applying a large potential difference across the accelerator tube. The energy of 1.6 MeV was used in this study. The acceleration voltage is generated from a moving insulating belt that carries charge that is sprayed on at base plate, and this charge is removed as a column current flowing through a set of resistors [5].
The dipole magnet in front of the Van de Graaff accelerator acts as an energy and mass separator. It deflects the beam into either the left beam line or right beam line. The Left Beam line is designed to operate below room temperature while the right beam line operates at room temperature. For this study, the right beam line was used. The beam of positively-charged ions (He+) passes through slits that stabilise the accelerating voltage, and they are collimated into a specific size before they reach the target. Secondary electrons are suppressed by a negative voltage of 200 V connected to a ring electrode in front of the target holder.
The backscattered alpha particles are detected by a Si surface barrier detector operating with a reverse bias of 40 V, and connected to a pre-amplifier where it is integrated into a voltage signal that is proportional to backscattered energy. This voltage signal is amplified before it is converted to a digital signal by an analogue to digital converter (ADC) inside the multi-channel analyzer (MCA). The MCA output is a spectrum of the yield (counts) versus channel number. The yield is the number of backscattered particles at 165°, while the channel number is proportional to the backscattered energy.

2.1 Introduction
2.2 Crystal structure of germanium
2.3 Band structure of germanium
2.4 Metal-semiconductor contacts
2.4.1 Introduction
2.4.2 Schottky barrier formation
2.4.4 Depletion layer
2.4.5 Image-force lowering of barrier
2.4.6 Ohmic contact
2.4.7 Current transport mechanisms Thermionic emission current Quantum-mechanical tunnelling Generation recombination current
2.4.8 Barrier height determination
2.4.9 Barrier height inhomogeneities
2.5 Annealing studies and germanide formation
2.6 Fundamentals of defects in semiconductors
2.6.1 Introduction
2.6.2 Vacancy defect
2.6.3 Interstitial defect
2.6.4 The E-centre and A-centre
2.7 Aspects of deep level transient spectroscopy
2.7.1 Introduction
2.7.2 Deep level transient spectroscopy
2.7.3 Emission and capture of carriers by trapping centres
2.7.4 Capacitance transient
2.7.5 Principles of deep level transient spectroscopy
2.7.6 Defect depth profiling
2.7.7 Principles of Laplace-DLTS
2.7.8 Field dependence of the emission rate
3.1 Introduction
3.2 Sample preparations
3.2.1 Germanium cleaning process
3.2.2 Ohmic and Schottky contact fabrication
3.3 Current-Voltage and Capacitance-Voltage measurements…
3.4 Deep level transient spectroscopy (DLTS) and Laplace-DLTS system
3.5 Rutherford backscattering spectrometry
3.6 Scanning electron microscopy
3.7 Annealing apparatus
4 RESULTS: Thermal annealing behaviour of metal Schottky contacts on n-Ge (100)
4.1 Introduction
4.2 Experimental procedures
4.3 Results
4.4 Discussions
4.5 Summary and conclusions
List of publications
5 RESULTS: Morphological evolution of metal Schottky contacts on n-Ge
5.1 Introduction
5.2 Experimental procedures
5.3 Results
5.4 Summary and conclusions
6 RESULTS: The barrier height distribution in identically prepared metal Schottky contacts on n-Ge (100)
6.1 Introduction
6.2 Experimental procedures
6.3 Results
6.4 Summary and conclusions
List of publications
7 RESULTS: Studies of defects induced in Sb doped Ge during contacts fabrication and annealing process
7.1 Introduction
7.2 Experimental procedures.
7.3 Results
7.4 Summary and conclusions
List of publications
8 RESULTS: Current-Voltage Temperature characteristics of n-Ge (100) Schottky barrier diodes
8.1 Introduction
8.2 Experimental procedures
8.3 Results
8.4 Summary and conclusions

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