GB1482929A - Apparatus and method for measuring carrier concentration in semiconductor materials - Google Patents

Abstract

1482929 Measuring capacitance; testing semi-conductor parameters POST OFFICE 14 May 1975 [16 May 1974(2)] 21902/74 and 21903/74 Heading G1U To measure a characteristic of a semi-conductor material which varies with depth, the material is placed in a cell including an electrolytic solution which forms a Schottky Lavrier rectifying contact at the semi-conductor surface and also is capable of electro-chemically etching the semi-conductor surface, while a parameter representing the characteristic is continuously measured. Electrolytic cell, Fig. 4 A PTFE tube contains a chamber 1 through which a KOH solution is passed from inlet 2 to outlet 3. A carbon electrode 12 and a porous-plug type electrode 13 containing calomel contact the KOH solution. An orifice 4 is closed by a sample of semi-conductor 5 e.g. GaAs or GaP and a sealing ring 7 prevents egress of solution while also defining a specific area 6 at which the Schottky diode is formed. A fibre optic light pipe 11 is situated opposite the orifice 4 to direct sharply filtered light from a tungsten-halogen lamp (not shown) on to the diode 6. The sample is held in place by a plunger 10 acting,on gold alloy springs 8, 9 which also serve as electrical contacts. A platinum wire 14 serves as a measuring electrode. Measurement circuitry, Fig. 7 The cell, represented at 31, is connected to circuitry which reverse biases the Schottky diode (below its breakdown voltage) and measures the depletion layer capacitance to determine the carrier concentration at the junction. As the KOH solution electro-chemically dissolves the semi-conductor material, a plot of the concentration profile is obtained. A circuit 37 controls the D.C. current through the cell which dissolves the semi-conductor, to maintain the potential at the calomel electrode (and hence the anodic potential) constant. The depletion layer capacitance is measured differentially by applying to the Schottky diode a signal comprising a 50 mV r.m.s. 3 kHZ signal from oscillator 38 with a 100 mV r.m.s. 30 Hz signal from Wien bridge oscillator 43 superimposed thereon. The capacitance is measured by sensing the quadrature component of the output current from the sample in a phase-sensitive amplifier 40. A similar amplifier 41 has a 30 Hz reference and this is used to compensate for zero errors. The output therefrom C+dC/A is passed to a divider, providing an output A## o /C‹dC which represents W D + dW D , the depletion layer width. The 30 H Z component representing dWD is squared, and detected to provide a value #/n, where n is the carrier concentration, and converted n 52 to a value log n which is used at the y-input to a recorder. The amount of material WR removed by the etching is proportional to the time integral of the dissolution current, and a signal proportional thereto is derived by meter 55 and integrator 57 which preferably is formed by a stepping motor to accomodate long integration times. The signal representing WR is then added to the signal WD representing the depletion layer width to provide a measure of the actual depth at which the measurement of log A is taking place. This forms the yinput to the recorder. A device 60 senses when the integrator has reached full-scale, and stops the measurement or recycles the integrator. As an alternative to a full-line plot, the integrator may indicate 360 equal steps over the selected full-scale, and at each step operate the recorder pen to mark the record. In a simplified embodiment Fig. 6, the depth is determined by the electrostatic voltage between the sample back face and the Lavrier diode and the capacitance is determined directly from a measure of the diode current. For n-type material, light is directed on to the diode surface to provide the minority carriers necessary for the etching. For p-type material, the light is not necessary for etching, but the voltage must be momentarily reversed for the measurement step.