CN101315957A - Method and device for forming PN junction on P-type mercury cadmium telluride material by laser processing - Google Patents

Abstract

The invention relates to a method and a device for forming a PN junction on a P-type mercury cadmium telluride material by laser processing. This method is to focus the pulsed laser on the surface of the P-type HgCdTe material, and irradiate the P-type HgCdTe material in a short time, so as to form a hole area ablated by the laser on the material, and the diameter of the hole is A few microns to more than ten microns; an inversion area, that is, an N-type area, is formed in the area of several microns around the hole, and the inversion area forms a PN junction area with the P-type area around the hole. The device of the invention is composed of a femtosecond laser, a deflection mirror, a neutral density filter, a pupil, an aperture, a dichromatic mirror, a lens, a CCD camera, a display and a workpiece platform. Due to the advantage of laser direct writing, the method of the present invention saves the photolithography process steps in the traditional junction technology, not only makes the junction process simple, but also helps to reduce the blind cell rate caused by the complexity of the process. The method is fully compatible with other existing technology in this field, such as readout circuit technology, and has direct practical value.

Description

Method and device for forming PN junction on P-type mercury cadmium telluride material by laser processing

technical field

The invention relates to a method and a device for forming a PN junction on a P-type mercury cadmium telluride material by laser processing, and belongs to the field of infrared photodetectors based on narrow band gap semiconductor mercury cadmium telluride materials.

Background technique

For a long time, mercury cadmium telluride (HgCdTe) has been widely used as a very important infrared photodetector material in military infrared thermal imaging, aerospace and satellite infrared remote sensing [Tang Dingyuan, Tong Peiming, Narrow Bandgap Semiconductor Infrared Detector , Semiconductor Devices and Research Progress II, Science Press, 1991]. In particular, photodiode array devices based on HgCdTe materials-infrared focal plane devices (including micromechanical scanning linear devices or staring focal plane devices) are currently one of the mainstream developments in the field of infrared detectors in the world [A .Rogalski, Infrared Detectors_Gordon and Breach Science, Amsterdam, 2000]. In this technical field, the PN junction technology that constitutes the detector chip is the core of the photodiode detector array technology. At present, the international methods mainly include ion implantation and ion etching [J.Rutkowski, Opto-Electron.Rev. 12, 123 (2004)]. In these traditional methods, photolithography is an indispensable process step, and a series of physical and chemical processes such as photolithography coating, development, and degumming will increase the probability of blind cells in the photosensitive element array, thereby affecting the quality of the device. rate has a negative impact.

Contents of the invention

The object of the present invention is to provide a method and device for forming a PN junction on a P-type HgCdTe material by laser processing in view of the defects in the prior art. The method of the invention not only effectively reduces the complexity of the process, but also reduces the probability of blind element rate caused by the complexity of the process, so that the method has unique advantages over the traditional method.

To achieve the above object, the present invention adopts the following technical solutions:

A method for laser processing to form a PN junction on a P-type HgCdTe material, which is characterized in that the pulse laser is focused on the surface of the P-type HgCdTe material, and the P-type HgCdTe material is irradiated in a short time, In this way, a hole area ablated by the laser is formed on the material, and the diameter of the hole is several microns to more than ten microns; an inversion area, that is, an N-type area, is formed in the area of several microns around the hole, and the inversion area and the P area around the hole are formed. Type region forms a PN junction region.

In the above method, the size of the hole is controlled by adjusting the laser power and the focal spot of the laser focus.

In the above method, the PN junction array area of the line array or area array is obtained by controlling the movement of the surface position of the workpiece to be processed.

A laser processing device for forming a PN junction on a P-type mercury cadmium telluride material, which is applied to the above method, includes a laser and a workpiece platform, and is characterized in that the laser beam emitted by the laser passes through two deflection mirrors, a center After a linear density filter, a pupil, a beam, and a dichromatic mirror, the laser beam refracted by the dichromatic mirror is radiated to the workpiece placed on the workpiece platform through a lens, and the beam passing through the dichromatic mirror is transmitted by the CCD camera to the display.

The above-mentioned light beam is connected to a computer, and the computer controls the opening interval of the light beam, thereby controlling the time for the laser beam to irradiate the surface of the workpiece.

The above-mentioned laser is a femtosecond laser, and the pulsed laser generated is a femtosecond laser.

The workpiece platform mentioned above is a piezoelectric ceramic displacement platform, and the displacement of the workpiece platform is controlled by piezoelectric ceramics.

In terms of physical principles, the method of the present invention is based on the specific role of mercury atoms in the HgCdTe material. It is theorized that mercury vacancies act as acceptors in HgCdTe materials, while interstitial mercury atoms act as donors. Mercury has a low binding energy in the material and is easy to diffuse. If the bond of mercury in a certain local area of the material is broken, the mercury atoms will rapidly diffuse to the surrounding area. This not only makes a large number of mercury vacancies that play the role of acceptors in the P-type material be reoccupied, but also greatly increases the number of interstitial mercury atoms that play the role of donors, which leads to the region (usually in the range of several microns) around the hole region. Sexual antitype. This mechanism is used to explain the inversion of HgCdTe material caused by ion etching [I.M.Baker and C.D.Maxey, J.Electron.Mater.30, 682_2001], and is also applicable to explain the laser etching proposed by the present invention to form a junction plan.

Compared with the prior art, the present invention has the following obvious outstanding substantive and significant advantages: the present invention utilizes a focused laser beam to bombard the surface of P-type HgCdTe material in a short time to form a hole area and form a PN junction, which can be directly Applied to the construction of infrared focal plane array devices. The method of the present invention has the advantage of laser true black, saves the photolithography process steps in the traditional junction technology, not only makes the junction process simple, but also helps to reduce the blind cell rate caused by the complexity of the process. The method is fully compatible with other existing technology in this field, such as readout circuit technology, and has direct practical value.

Description of drawings

Figure 1 is a schematic diagram of the structure of a PN junction device formed on a P-type HgCdTe material by laser processing.

Fig. 2 is an optical micrograph of the ring hole array formed on the P-type HgCdTe material by laser processing.

Figure 3 is the detection principle of the PN junction by the laser induced current spectrometer.

Figure 4 is the detection result of the laser-induced current Puyi on the laser-etched PN junction array.

Detailed ways

A preferred embodiment of the present invention is described in detail as follows in conjunction with accompanying drawing:

The method of laser processing in this example to form a PN junction on the P-type HgCdTe material is: focus the pulse laser on the surface of the P-type HgCdTe material, and irradiate the P-type HgCdTe material in a short period of time. In this way, a hole area ablated by the laser is formed on the material, and the diameter of the hole is several microns to more than ten microns; an inversion area, that is, an N-type area, is formed in the area of several microns around the hole, and the inversion area and the P area around the hole are formed. Type region forms a PN junction region.

The experimental sample in this example is a HgCdTe thin film material grown on a CdZnTe substrate by liquid phase epitaxy. The thickness of the epitaxial film is 24 μm, and the surface is covered with a 0.25 μm ZnS passivation layer. The Hall test gave a carrier concentration of P=2.4×10 ¹⁶ cm ⁻³ .

Referring to Fig. 1, the device that the laser processing of this example forms PN junction in P-type mercury cadmium telluride material comprises a laser (3) and a workpiece platform (12), it is characterized in that the laser beam that laser (3) emits passes through two successively After deflection mirrors (4, 5), a neutral density filter (6), a pupil (7), a diaphragm (8), and a dichromatic mirror (9), the laser beam refracted by the dichromatic mirror passes through a The lens (10) radiates to the workpiece (11) placed on the workpiece platform (12), and the light beam passing through the dichroic mirror is captured by the CCD (1) and transmitted to the display (2).

The laser (3) used is a titanium sapphire laser with an output pulse width of 120 fs, a wavelength of 800 nm, and a maximum output energy of 600 μJ. The neutral density filter (6) in the optical path is used to adjust the laser beam current density, and the pupil (7) also plays a role in adjusting the luminous flux. The time of one light irradiation is controlled by the opening of the diaphragm (8) The interval is determined. The two-color deflection mirror (9) in the optical path enables the CCD optical monitoring system to monitor the position of the spot on the sample surface, and on the other hand, it can effectively suppress the interference of the 800nm strong laser spot reflected by the sample. The sample is located on the workpiece platform (12), and the workpiece platform (12) is a piezoelectric ceramic displacement stage whose displacement is controlled by piezoelectric ceramics (PZT), and the displacement precision of the sample reaches 20nm. The position of the laser spot on the surface of the sample is directly controlled by the computer program, so that a single etching hole can be generated on the sample, or a series of photoetching positions can be preset to construct an array of etching hole patterns. Figure 2 shows the topography of the optical microscopic image of the part of the laser-etched hole array. The experimental data show that both laser intensity and radiation time have a significant impact on the etching morphology. The laser etching energies corresponding to the three rows of holes from top to bottom in Fig. 2 are 100mW, 80mW and 50mW respectively.

Whether the etched hole corresponds to the PN junction and the strength of the photoelectric response of the PN junction can be checked by a laser-induced current (LBIC) test. The detection method and principle are shown in Figure 3. Build a pair of electrodes that form ohmic contact with the base material at both ends away from the PN junction structure, and connect the pair of electrodes in series with the ammeter to form a closed loop, as shown in Figure 3. When the focused laser beam is scanned relative to the sample, if the laser beam is incident on the PN junction region, the photogenerated carriers will be separated by the built-in electric field of the PN junction, generating a photoelectromotive force, which will correspondingly cause an induced current in the closed loop, The magnitude of this photo-induced current reflects the strength of the photovoltaic effect of the PN junction. For the n-on-p structure formed on the P-type substrate shown in Fig. 3, the n region and the p regions on the left and right sides form a pair of PN junctions with opposite built-in electric fields. When the light beam sweeps across the n region, a photoresponse structure in which the polarity of the current changes is shown on the obtained photocurrent/spot position relationship curve, and the two photocurrent response peaks correspond to the interface of the PN junction region. Figure 3 schematically shows the photocurrent/position curve of LBIC for an n-on-p PN junction structure.

The test results of LBIC on the PN junction formed by the laser etching method proposed by the present invention are shown in FIG. 4 . The three sets of curves from left to right in the figure are the LBIC test results of the hole area produced by etching with several different laser pulse powers of 100mW, 80mW and 50mW in an etching time of 5 milliseconds (see the attached figure for the etching geometry 2). It can be seen that the three kinds of etched holes all present obvious PN junction light corresponding characteristics. Since the LBIC laser spot diameter we use is only about 1 μm, which is far smaller than the size of the junction area, the excitation spot can be approximately regarded as a point light source, and the change range of the entire curve can be approximately regarded as the spatial size of the photovoltaic response area. From the spatial scale indicated by the double arrows in the figure, although the etching laser pulse power of several holes is different, the scale of the PN junction area or the effective scale of the photosensitive element is very close, and the value is 25-26 μm. In addition, the laser engraving The etching power has a certain influence on the photovoltaic response intensity of the junction, and the response intensity shows an obvious increasing trend according to the etching power of 50mW, 80mW, and 100mW. In addition, the experiment also found that when the etching time is longer than tens of milliseconds, the junction electric field will have obvious asymmetry. From our experimental results, the etching power is selected to be 50-80mW, and the etching time is 5 Milliseconds can give a more optimized knotting effect.

Claims (7)

1. a laser processing forms the method for PN junction on P type mercury cadmium telluride, it is characterized in that the surface of pulse laser focusing to P type mercury cadmium telluride, at short notice P type mercury cadmium telluride is carried out irradiation, thereby form by the hole district of laser ablation on this material, the diameter of hole is several microns to tens microns; Form inversion regime in several microns zones of hole periphery, i.e. N type district, PN junction district of p type island region formation of this inversion regime and hole periphery.

2. laser processing according to claim 1 forms the method for PN junction on P type mercury cadmium telluride, it is characterized in that controlling by the focal spot of regulating laser power and laser focusing the size of hole.

3. laser processing according to claim 1 forms the method for PN junction on P type mercury cadmium telluride, it is characterized in that obtaining the PN junction array district of linear array or face array by controlling the surface location of mobile workpiece to be machined.

4. a laser processing forms the device of PN junction on P type mercury cadmium telluride, being applied to laser processing according to claim 1 forms in the method for PN junction on P type mercury cadmium telluride, comprise a laser (3) and a work piece platform (12), the laser beam that it is characterized in that laser (3) ejaculation is successively through two deflecting mirrors (4,5), a neutral-density filter (6), a pupil (7), a diaphragm (8), behind the dichroic mirror (9), be subjected to the laser beam of dichroic mirror refraction to be radiated the workpiece (11) that places on the work piece platform (12), and be transferred to display (2) by CCD (1) shooting through the light beam of dichroic mirror through lens (10).

5. laser processing according to claim 4 forms the method for PN junction on P type mercury cadmium telluride, it is characterized in that described diaphragm (8) connects a computer, by the unlatching blanking time of computer control diaphragm, thereby the control laser beam is to the time of surface of the work irradiation.

6. laser processing according to claim 4 forms the method for PN junction on P type mercury cadmium telluride, it is characterized in that described laser (1) is a femto-second laser, and the pulse laser of generation is a femtosecond laser.

7. laser processing according to claim 4 forms the method for PN junction on P type mercury cadmium telluride, it is characterized in that described work piece platform (12) is piezoelectric ceramic displacement platform, by the displacement of piezoelectric ceramic control work piece platform.

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