In order to test the material quality of the CZT grown at IMEM-CNR institute, several de-tector samples were made during this work. The aim of these characterizations was to study different properties of the grown material: resistivity, metal-semiconductor contact, transport properties and spectroscopy performance. For testing the material, ingots were cut and simple detectors with planar electrodes were prepared and characterized. The detector preparation will be discussed also in chapter 6.
5.3.1 Cutting
The 2 inches ingots grown at IMEM institute ( shown in fig.5.2 ) are normally cut into wafers perpendicular to the growth axis. In this way the effect of Zn segregation (that results in a not-uniform concentration of Zn along the growth axis) can be reduced. Several ingots were first
cut into wafers and than cut again into smaller samples with rectangular shape. The cutting process is ideally a trivial procedure, but on the contrary it is one of the critical steps in the realization of CZT devices. Every cut, in fact, produces several damages that may have a large influence on the performances of the sample as a radiation detector. For the wafer cutting a diamond saw is used; for cutting the wafer into smaller samples a diamond wire saw is used. The diamond saw for cutting ingot is a standard machine for semiconductor industry with a 0.3 mm thick diamond circular blade. The wire saw is a South Bay Tech. machine that allows a fine regulation of the weight to be applied on the wire and by means of this tool an extremely soft cut can be made.
The samples used for the characterization of the ingots have usually areas of 7 mm x 7 mm or 5 mm x 5 mm, therefore many devices can be obtained from a single wafer. Before cutting wafers optical and IR transmission inspections are normally carried out in order to select the usable parts of wafers.
5.3.2 Mechanical polishing
The obtained samples are then mechanically polished. The aim of this process is to remove the parts of the crystal that were damaged during the cut. Normally a 100-150 mm layer of material must be removed in order to remove all the damaged parts.
Polishing technique consisted of a rotating plate where sandpapers with different grains were mounted. The system was equipped with bi-distilled water supplier in order to have the best lu-brication on the sandpaper without introducing any type of impurities. We developed a standard procedure for mechanical processing that is working very well with CZT crystals.
• In the first step a paper with relatively large grain (p2500, 8.4 µm of average particle diameter) was used. The aim of this first stage was to remove a relative thick layer of material in order to eliminate the existing contacts and eventually the scratches present on the surfaces.
• In the second step p4000 SiC grains sandpaper (5 µm of average particle diameter) was used. This step is fundamental for subsequent machining, with p4000 all the remaining scratches must be eliminated. For this reasons this step is the longest one.
• After the lapping (process with sandpaper), the polishing procedure started. The sandpa-per was replaced by polishing cloths. A set of abrasive diamond suspension was used with diameters ranging from 3µm down to 0.1 µm. The entire process required a usually one day for the two sides of each sample.
• After the polishing and the mechanical polishing the sample was cleaned with two hot solvents: acetone (10 minutes at boiling temperature) and isopropanol (10 minutes at boiling temperature).
At the end of the polishing process the samples have mirror surfaces. At the end of this process the samples are cleaned with hot solvents (boiling temperature): toluene, isopropanol and acetone.
5.3.3 Contact deposition
The development of a reproducible and high performance contact deposition technique met some difficulties. The main problems are connected with CZT characteristics. CZT is fragile, especially near the edges, and a particular care has to be taken when techniques, like photolithography that requires contact between the sample and the mask, with a consequent pressure on the sample, are used. Moreover the poor adhesion of metals on the CZT surface is well known. This limit cannot be overcome with a thermal annealing, as it’s usually done for other materials, because of the detrimental change in CZT properties with temperatures higher then 100-150 °C.
At IMEM-CNR institute the electrical contacts on CZT are deposited using a wet technique called “electroless depostion”. In this technique an aqueous solution of HAuCl4 (5%) is used to deposit a thin layer of gold on CZT surface.
When HAuCl4 salt is in an aqueous solution, the following reaction takes place:
HAuCl4+ H2O⌦ H[AuCl3OH] + HCl
In particular, without any external current, two reactions (one anodic and the other cathodic) take place simultaneously at the surface of the CZT crystal. The complete reaction is:
2H[AuCl3OH] + CdZnT e⌦ 2Au + CdCl2+ ZnCl2+ T eCl4+ 2H2O
In this reaction Au atoms are deposited on CZT surface and CdCl2, ZnCl2and T eCl4salts are dissolved in the solution[44].
The thickness of the gold layer roughly increses as the square root of the time of reaction and the typical deposited thickness is around 600 Å[45]. .
In this type of process a significant amount of gold diffuses into the bulk following the same square root law creating a thin highly doped region under the contacts. Due to the low doping efficiency of gold, however, perfect ohmic contacts cannot be achieved by using this method on CZT. Electroless contacts always exhibit a small rectifying behaviour.
At IMEM-CNR institute the standard deposition is obtained with 1 min of reaction at 20°C.
Using this procedure a good reproducible contact can be deposited on CZT crystals. The typical IV characteristics of these contacts are shown in fig.3.5.
5.3.4 Passivation
In order to increase the surface resistivity, a passivation process must be done after the deposition of contacts. The standard passivation procedure is done using an aqueous solution of NH4F + H2O2(3%). For this process, the sample is immersed for 6 minutes in room temperature solution.
After this process, the reduction of leakage currents can be ascribed the formation of an oxide layer with higher resistivity. The oxide layer formed by NH4F-based etching was studied by optical ellipsometry with a variable angle spectroscopic ellipsometer working in the range 200-1700 nm. The measured refractive index for the layer is compatible with that of an oxide resulting from a mixture of ZnO, T eO2 and CdO ( n=2.2 and n=2.5 at 1500 nm, respectively).
The passivation procedure can be performed on the sample before or after the contact deposition.
It has been tested that the oxidizing solution does not affect or remove the metal layers on the CZT surface.