In order to evaluate the chemical purity of scCVD diamond films, several randomly chosen samples were characterized by spectroscopic techniques: in the infrared (IR), visible (VIS) and ultraviolet (UV) range as well as by electron spin resonance (ESR) and finally by total photoelectron emission yield spectroscopy (TPYS).
Nitrogen impurity: Nitrogen incorporated into the diamond lattice, due to five valence electrons, becomes a deep donor with an ionization energy of 1.7 to 2 eV. Therefore it
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appear isolated or, alternatively, form aggregates. Such defects form various electrically active states within the diamond band-gap, giving rise to absorption in the VIS or the IR range.
• The single substitutional nitrogen (called type C center) was observed in all synthetic diamonds, HPHT and CVD crystals. It gives rise to a characteristic IR spectrum with a broad peak around 1130 cm−1 and a sharp local mode peak at 1344 cm−1 [Zai01].
As a non-ionized donor at RT, it gives rise to a continuous absorbtion in UV-VIS spectroscopy starting at ∼2 eV. As element with unpaired valence electrons it gives rise to the P1 ESR signal [Nok01].
• The nearest neighbor pair of substitutional nitrogen atoms, called the A aggregate, is present in the majority of natural diamonds (type IaA) and can be also incorporated into CVD films in form of N2 molecules [Neb00]. Its IR spectrum reveals a major peak at 1282 cm−1 and another one at 1212 cm−1 [Zai01].
Figure 4.1 (Left panel) shows an ESR spectrum measured with two scCVD samples:
a) a high purity IIa diamond and b) a diamond produced by E6. The concentration of P1 centers was about ∼1015 cm−3 for the IIa sample (top spectrum). Despite of a 15 times longer scan for the E6 sample, no detectable ESR signal could be registered (bottom spectrum). The conclusion is that the concentration of the single-substitutional nitrogen within this sample is below 1014 cm−3, i.e., the detection limit of the ESR method.
Examples of IR absorbtion spectra of three synthetic diamonds are shown in Figure 4.1 (Right panel). Characteristic nitrogen absorbtion bands (A and C centres) in the one-phonon region can be observed for two HPHT diamonds (black and red curves). No signa-ture of nitrogen impurity in form of A or C centers is found for the E6 scCVD diamond, within the sensitivity of the IR absorption method, (i.e., 1 ppm).
Boron impurity: Boron is present in the very rare type IIb natural diamonds. In CVD diamond growth, boron is deliberately added in order to obtain p-conductive films [Col79].
However, due to pollution from the steel chambers of the growth reactors, unwanted con-tamination with boron can occur during the growth process even of intrinsic diamond films.
If boron, containing three valence electrons, is incorporated into the diamond lattice, it behaves approximately as a ’hydrogenic’ acceptor with relatively low ionization energy of 0.368 eV. It gives rise to photoconductivity above this energy threshold. The diamond appears bluish due to absorbtion in the red region of the visible light spectrum. Boron acts as a shallow trapping center for excess charge carriers, affecting the operation of diamond detectors negatively.
Figure 4.2 presents the results of total photoelectron emission yield spectroscopy (TPYS) of a scCVD produced by E6 (black curve) and a high quality IIa CVD diamond (red curve). The measurement was carried out at 300 ℃. At this temperature, most of the boron acceptors are ionized, gaining electrons from the valence band (VB). The band-gap at 573 K is estimated from Eg = 5.48 eV-∆E(T ) = 5.48 eV - 0.06 eV = 5.42 eV.
By subtracting the energy level of the boron acceptor from the bang gap, one gets the energy difference of the electron transition from the acceptor level to the CBM, which is
4.2. Atomic Impurities in scCVD Diamond 39
Figure 4.1: (Left panel) ESR spectra of two scCVD diamonds a) a high purity IIa diamond and b) a scCVD diamond sample produced by E6 (courtesy Ch. Nebel). No detectable ESR signal was found for the E6 sample within the sensitivity range of the method of ˜1014cm−3. (Right panel) IR absorption spectra of three synthetic diamonds, showing absorption in the one-phonon range by nitrogen impurities (A and C centers) in a HPHT diamond (red and black curves). No absorption could be detected for the E6 scCVD diamond (blue curve).
∆E = 5.06 eV. This transition is marked as I in the band diagram of Figure 4.2 (Right panel). The next onsets above 5.2 eV are due to exciton ground states and the absorption edge with phonon-absorption at 573 K, (marked in the band diagram as II ). For both samples, boron related excitation is present. However, in the case of the E6 sample, the measured photoelectron emission yield is about one order of magnitude smaller. From cross calibration with a sample of known boron impurity concentration, the amount of boron for the E6 sample was estimated to≤ 1015 cm−3.
Figure 4.2: (Left panel) TPYS spectra of two scCVD diamonds measured at 300℃ (courtesy Ch. Nebel). In both cases onset at ∼ 5.06 eV is visible suggesting, transition from the boron impurity level (0.36 eV) to CBM as indicated in the band diagram (Right panel).
spectrum of an HPHT Ib (containing nitrogen impurities) is displayed in black, where the absorbtion starts around 2 eV (photo-ionization onset of nitrogen impurities). No absorbtion is registered for the scCVD diamond up to the fundamental edge absorption at the band gap energy of diamond ∼5.46 eV (at RT). The right graph of Figure 4.3 presents the edge absorption of a scCVD diamond in expanded energy scale. Since, diamond is an indirect semiconductor, three thresholds in the absorption spectrum can be distinguished.
The first and the second threshold correspond to the creation of an exciton (Egx= 5.406 eV) with the absorption of a transverse optic (TO) or a transverse acoustic (TA) phonon, respectively, whereas the third threshold corresponds to the creation of an exciton with the emission of a TA phonon. The absence of absorbtion in the near red region of visible light and an edge absorption with exciton creation is typical only for high purity IIa diamonds.
The UV - VIS absorption spectra of about six scCVD samples measured have shown the same characteristic.
Figure 4.3: (Left panel) Absorption spectra in the VIS - UV range of an intrinsic scCVD and an Ib HPHT diamond measured at room temperature. (Right panel) Fundamental absorbtion edge at the band-gap energy of e6 scCVD diamond. The thresholds (1) and (2) involve creation of an exciton with the absorption of a TO or a TA phonon, and threshold (3) involves the emission of a TA phonon.